the umd/nist fast neutron spectrometers
TRANSCRIPT
The UMD/NIST Fast Neutron Spectrometers
T.J. Langford Yale University
E.J. Beise, H. Breuer University of Maryland
C.R. Heimbach, J.S. Nico National Institute of Standards and Technology
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Outline• Fast neutrons for low-background science
• At sea-level
• Underground
• Previous detection techniques
• Capture gated spectroscopy
• The FaNS detectors
• FaNS-1: The proof-of-principle
• FaNS-2: The purpose-built detector
• Outlook for FaNS
T.J. Langford WIDG Seminar 2/18/2014���2
Fast Neutrons for Low-Background Physics
T.J. Langford WIDG Seminar 2/18/2014���3
Fast Neutrons for Low Background Science
• Fast neutrons play a particularly problematic role in low background experiments
• Deeply penetrating, and difficult to shield
• Surface → Activation of shielding and detector materials
• Backgrounds for experiments that must run sea-level
• Underground → FNs are indistinguishable from WIMP dark matter interactions
• Important to know the fast neutron spectra in both environments for experiment design and background rejection
T.J. Langford WIDG Seminar 2/18/2014���4
Sea-level Cosmogenic Neutrons• Very high energy neutrons are
created in cosmic ray air showers
• Low background materials are required for many experiments, especially (Ge, Pb, Cu)
• Determination of material activation requires precision knowledge of neutron flux and spectrum
• Large variations among measurements and simulations
• Large fluctuations with local environment and conditions
T.J. Langford WIDG Seminar 2/18/2014
3432 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 6, DECEMBER 2004
TABLE IIFITTED PARAMETERS FOR THE ANALYTIC MODEL
. In the Appendix, we provide a table of numer-ical values of neutron differential flux, , fordetermined from the shape of the Yorktown Heights spectrumscaled down to the location of New York City (NYC) at sealevel (as it is shown in Fig. 6), then further adjusted up to the fitin Fig. 5, and up to the mid-point of solar modulation using (3).We refer to this spectrum as the “reference” spectrum. Its flux is0.901 times that of the measured spectrum at Yorktown Heightsand 1.07 times that of the scaled spectrum shown in Fig. 6.
The spectrum may change shape slightly in the GeV region athigher cutoffs. The airplane measurements of Goldhagen et al.[14], obtained at an altitude of 20 km, found that the fraction ofthe total flux that was above 10 MeV was 8% higher at a cutoffof 11.8 GV than at 0.8 GV. Calculations by Mares et al. [37]show that the cutoff dependence of the spectrum shape is lessthan half as much on the ground as it is at 20 km.
V. ANALYTIC MODEL
In addition to tabulated values of , a simple analyticexpression has been fit to the NYC reference neutron spectrumin the energy range from 0.1 MeV to 10 GeV
(6)
The values of the parameters , , and in (6) wereconstrained by requiring that the energy-integrated fluence,
, in the energy regions of the evaporation andhigh-energy peaks from the model agree with the experimentaldata to within a few percent. The numerical values of theseparameters are listed in Table II. Since the flux in the thermalpeak, and to a lesser extent the plateau region, depend onthe local environment, functions fitting these regions are notpresented here.
Fig. 7 shows a graph of the evaporation and high-energy re-gions of the reference spectrum and the analytic model in the
representation. The data are shown as a histogramand the analytic model is the solid smooth curve. The fit is vis-ibly very good above 10 MeV and reasonably good in the evap-oration region down to about 0.4 MeV.
VI. COMPARISON TO JEDEC STANDARD
Fig. 7 also shows the spectrum from Appendix E of JEDECStandard JESD89 [38]. The JEDEC model was a fit throughpreviously published data adjusted to the same conditions asour reference spectrum. (This was the main reason we chosethose conditions.) It has been used for a number of years andforms the basis of current SER calculations. The JEDEC modelunderestimates the reference measured flux integrated from 50MeV to 1 GeV and overestimates
Fig. 7. Upper-energy portion of the reference neutron spectrum at New YorkCity, sea level, and mid-level solar modulation (histogram), the analytic fit (solidsmooth curve), and the model from Appendix E of JEDEC Standard no. 89 [38](dashed curve).
Fig. 8. Differential flux, , of cosmic-ray inducedneutrons as a function of neutron energy. The data points are our referencespectrum from the measurements, the solid curve is our analytic model, andthe dashed curve is the JEDEC model [38].
it from 5 to 50 MeV and again from 1 to 10 GeV by factors of1.7 and 1.3, respectively.
Fig. 8 shows the same data as Fig. 7, but presented as themore familiar differential flux (neutrons ).As already shown in Fig. 7, our analytic model reproduces themeasured spectrum better than the JEDEC model.
VII. CONCLUSION
Five sets of neutron spectrometer data have been collectedand analyzed from a variety of sites across the United Statesto determine the flux and energy distribution of cosmic-ray in-duced neutrons on the ground. The measurement sites had awide range of altitudes and a small range of geomagnetic cutoffrigidities (1.6 to 4.7 GV). An extended-energy Bonner spherespectrometer was used that collected data simultaneously acrossan energy range from 1 meV to about 10 GeV. The measuredtotal neutron flux varied by a factor of 15 from the highestto lowest altitude sites, but the shape of the spectrum above
Gordon et al. (2004) FN Spectrum with 14
Bonner Spheres
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Sea-level Cosmogenic Neutrons• Very high energy neutrons are
created in cosmic ray air showers
• Low background materials are required for many experiments, especially (Ge, Pb, Cu)
• Determination of material activation requires precision knowledge of neutron flux and spectrum
• Large variations among measurements and simulations
• Large fluctuations with local environment and conditions
T.J. Langford WIDG Seminar 2/18/2014
- 3 measurements with the same detector
- 3 very different fluxes
0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
(E x
Flu
x) (
n/cm
2 /s)
101 102 103 104
Energy (MeV)
Ziegler 1996 Gordon 2004 CRY Simulation
3 different FN spectra
���6Nakamura et al. 2005
Underground Neutrons
T.J. Langford WIDG Seminar 2/18/2014
• 1-10MeV: Natural radioactivity from surround material, mainly (alpha,n) (location dependent)
• 1 MeV - GeV: Muon-induced spallation neutrons (depth dependent)
• Addition of shielding materials (like Pb) can enhance the production of muon-induced neutrons
15
events/keV-kg-year, is precisely what we have found inour simulation.
Depth (km.w.e.)1 2 3 4 5 6
)-1 y
ear
-1 k
g-1
Even
ts (k
eV
-710
-610
-510
-410
-310
-210
-110
12510
2610
2710
2810
Hal
f Life
(yea
rs)
Majorana
Granularity, PSD and Segmentation
Target sensitivity
Without neutron veto
With neutron veto (99%)
WIP
P
Gra
n Sa
sso
Sudb
ury
FIG. 25: The Depth-Sensitivity-Relation (DSR) derived fora Majorana-like experiment showing, specifically, the resultsfrom this work assuming the detector is operated at a depthequivalent to the Gran Sasso Laboratory. The raw event ratein the energy region of interest of 0.026 events/keV-kg-yearcan be reduced by a factor of 7.4 by exploiting the detectorgranularity, pulse-shape discrimination (PSD), and detectorsegmentation. The upper curve displays the background sim-ulated in the case that no active neutron veto is present andthe lower curve indicates the reduction that would ensue if anactive neutron veto were present that is 99% efficient.
The results of our simulations can be used to derivethe DSR for Majorana as shown in Fig. 25. The neu-tron induced background can be reduced by about a fac-tor of 7.4 in Majorana owing to the use of crystal-to-crystal coincidences and the use of pulse-shape discrim-ination and segmentation. Nonetheless, to achieve thetarget sensitivity of next generation double-beta decayexperiments, 0.00025 events/keV-kg-year correspondingto the background level required to reach sensitivity tothe atmospheric mass scale of 45 meV Majorana neutrinomass, the muon-induced background must be reduced byroughly another factor of 100. This can be achieved onlyby operating such a detector at depths in excess of 5km.w.e., otherwise an active neutron veto would need tobe implemented with an efficiency in excess of 99%.
D. (α,n) Background
Once the depth requirement is satisfied, a proper shieldagainst (α,n) neutrons from the environment becomesnecessary. We use the standard rock and the measuredneutron flux (3.78×10−6cm−2s−1 [68, 69] ) at Gran Sassoassuming that all underground labs have the same orderof neutron flux to establish the shielding requirement for(α, n) neutrons. This flux corresponds to an average ofabout 2.63 ppm 238U and 0.74 ppm 232Th activity inGran Sasso rock and 1.05 ppm 238U and 0.67 ppm 232Thactivity in Gran Sasso concrete [70]. The neutron energy
spectrum depending on the rock composition is shown inFig. 26. As can be seen, the total neutron flux is aboutthree orders of magnitude higher than that of neutronsfrom the rock due to muon-induced processes, but theenergy spectrum is much softer.
Neutron Energy (MeV)0 1 2 3 4 5 6 7 8 9 10
)-1 M
eV-1
s-2
Neu
tron
s (c
m
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
0.22-610×
Standard Rock
Gran Sasso Rock
FIG. 26: The neutron energy spectrum arising from (α,n)reactions due to radioactivity in the rock. We predict a harderenergy spectrum in Gran Sasso rock relative to standard rockowing to the presence of carbon and magnesium.
To demonstrate the neutron flux and energy spectrumat different boundaries we show the rock/cavern neutronflux and energy spectrum with a shielding for Majoranadescribed earlier in Fig. 27 for the depth of Gran Sassoas an example.
Energy (MeV)-110 1 10 210 310
)-1 M
eV-1
y-2
Neu
tron
s (c
m
-1210-1110
-1010
-910
-810
-710
-610
-510
-410
-310
-210
-1101
10
210
310:Rock/Cavern boundaryµ
:After poly shieldingµ
:After lead + copper shieldingµ
,n): Rock/Cavern boundaryα(
,n): After poly + lead + copper shieldingα(
FIG. 27: The energy spectrum for fast neutrons producedby (α, n) reactions in the rock compared to those induced bymuon interactions in the rock with and without shielding. Thelower energy neutrons (< 10 MeV) are quickly absorbed usingpolyethylene shielding, however, the high energy portion ofthe muon-induced neutron flux persists. The addition of leadshielding adjacent to a detector can also create an additionalsource of muon-induced neutrons.
Note that the (α, n) neutrons from the rock are quicklyattenuated to the level of the muon-induced neutrons be-
(↵, n)
8
the corrected neutron multiplicity (equation (10)) andthe muon fluxes and distributions outlined in Section II.The neutron flux (φn) as a function of depth is shownin Fig. 14 where we have included a fit function of thefollowing form:
φn = P0(P1
h0)e−h0/P1 , (13)
where h0 is the equivalent vertical depth (in km.w.e.)relative to a flat overburden. The fit parameters are P0 =(4.0 ± 1.1)×10−7 cm−2s−1 and P1 = 0.86 ± 0.05 km.w.e..
Depth (km.w.e.)2 3 4 5 6
)-1 s
-2N
eutr
on F
lux
(cm
-1110
-1010
-910
-810
WIPPSoudan
Kamioka
Boulby Gran Sasso
Sudbury
FIG. 14: The total muon-induced neutron flux deduced for
the various underground sites displayed. Uncertainties on
each point reflect those added in quadrature from uncertain-
ties in knowledge of the absolute muon fluxes and neutron
production rates based upon our simulations constrained by
the available experimental data.
In Table V we summarize the neutron flux at therock/cavern boundary for the various sites consideredand note that we have not included the effect of neutronsthat emerge from one surface and back-scatter back intothe cavity. The results are in good agreement with theexisting simulation results for Gran Sasso [42]. If thesimulation results for Boulby [65] are modified using ourneutron multiplicity correction, good agreement is alsofound between the two results. It is relevant to note thatthere is a significant fraction of the neutrons with energyabove 10 MeV.
TABLE V: The muon-induced neutron flux for six sites (in
units of 10−9 cm−2s−1). The total flux is included along with
those predicted for neutron energies above 1, 10, and 100
MeV.
Site total > 1.0 MeV > 10MeV > 100MeV
WIPP 34.1 10.78 7.51 1.557Soudan 16.9 5.84 4.73 1.073Kamioka 12.3 3.82 3.24 0.813Boulby 4.86 1.34 1.11 0.277Gran Sasso 2.72 0.81 0.73 0.201Sudbury 0.054 0.020 0.018 0.005
2. Neutron Production in Common Shielding Materials
Fast neutrons can also be created by muons passingthrough the materials commonly used to shield a detectortarget from natural radioactivity local to the surroundingcavern rock. Fig. 15 shows the neutron yield in somecommon shielding materials. We have also included asimulation for germanium which will prove useful laterin this paper when we consider the DSR for experimentsbased on this target material.
Depth (km.w.e.)2 3 4 5 6
)-1
s-3
Neu
tron
Pro
duct
ion
Rat
e (n
cm
-1310
-1210
-1110
-1010
-910
-810 WIPPSoudan
KamiokaBoulby
Gran Sasso
Sudbury
LeadPolyethyleneCopperGermanium
FIG. 15: The muon-induced neutron production rate pre-
dicted for some common detector shielding materials. Note
that minor variations due to neutron back-scattering have
been neglected in these calculations.
The fitted functions have the same form as equa-tion (13) but with different values for parameters whichare provided in Table VI. To convert the neutron produc-tion rate to the total neutron flux, one multiplies equa-tion (13) by the average muon path length which dependsupon the detector geometry.
TABLE VI: Summary of the fitting parameters describing the
muon-induced neutron production rate in common detector
shielding materials.
Material P0 P1
Lead (7.84±2.21) × 10−8 0.86±0.05
Polyethylene (6.89±1.95) × 10−9 0.86±0.05
Copper (2.97±0.838) × 10−8 0.87±0.05
Germanium (3.35±0.95) × 10−8 0.87±0.05
Generally speaking, muon-induced neutrons producedin a detector target or surrounding shield can be activelyvetoed in coincidence with the primary, depending on theveto efficiency and specific detector geometry. Specificexamples are provided later in this paper.
Energy (MeV)-110 1 10 210 310
)-1 M
eV-1
y-2
Neu
tron
s (c
m
-1210-1110
-1010
-910
-810
-710
-610
-510
-410
-310
-210
-1101
10
210
310:Rock/Cavern boundaryµ
:After poly shieldingµ
:After lead + copper shieldingµ
,n): Rock/Cavern boundaryα(
,n): After poly + lead + copper shieldingα(
3
N’
µ µ ’
γ
n,π
N
(a)
γ
n,π
NN’
(b)
FIG. 1: Feynman diagrams depicting muon spallation (a) and photo-absoprtion (b). In panel (a)
an incoming muon exchanges a virtual photon with a nucleus. Panel (b) shows a real photon being
absorbed by the nucleus. The nucleus subsequently emits neutrons or pions.
the e↵ects of secondary processes, such as neutrons produced via primary pions or primary
neutrons.
II. MUON SPALLATION THEORY
In muon spallation, an incoming muon interacts with a nucleus via an exchange of a
virtual photon. The nucleus subsequently emits neutrons (or pions). Figure 1(a) shows a
Feynman diagram depicting this scenario. The di↵erential cross section for the inclusive
reaction of a charged lepton (see, for example, Ref. [7]), such as the muon, is
d�
dEf
d⌦f
=↵2
q4
pf
pi
Lµ⌫Wµ⌫
, (1)
where ↵ is the fine structure constant, pi
and pf
are the initial and final lepton momenta,
respectively, q is the four-momentum transfer, Lµ⌫ is the lepton tensor, and W µ⌫ is the
hardronic structure tensor. The lepton tensor is known analytically,
Lµ⌫ = 2⇥pµ
i
p⌫
f
+ p⌫
i
pµ
f
+ gµ⌫(m2 � pi
· pf
)⇤, (2)
where m is the lepton mass. The hadronic tensor, on the other hand, depends on nuclear
details that are not know analytically and must be empirically parametrized. It does satisfy
certain symmetries, however, which constrains the parametrization. For example, it must
���7Plots from Mei and Hime 2006
Neutron Production Simulation vs Data
Finally, it is apparent that the experimental pointslie far above the MC data. If the disagreement forthe light materials is already unreasonable, thecase of lead cannot be easily explained. In fact,even if a much lower neutron threshold of 1MeVis considered, both MC models would still predicta lower cross-section than those reported inRef. [18].
The production of lower energy neutrons bydeep inelastic scattering (DIS) of 470GeV muonsin lead was studied by the E665 Collaboration,who found average neutron multiplicities per DISevent of ’ 5 for neutron energies under 10 MeV[19]. A value of 3.7 is obtained for the GEANT4simulation of the m–N process at this muon energy,in reasonable agreement with the experimentalresult. The simulated neutron spectrum (per unitenergy) exhibits a double-exponential behaviourbelow 10MeV—also in agreement with the experi-mental findings. The two decay constants, due toneutron evaporation from the thermalised nucleusand from pre-equilibrium emission, are charac-
terised by nuclear temperatures of 0.93 and3.7MeV, compared to 0:7! 0:05 and 5! 1MeVobtained in E665. In conclusion, the spallation ofneutrons under 10MeV as predicted by GEANT4for lead does not conflict with these experimentaldata.The role of the minimum energy transfer in the
muon photonuclear models in neutron productionwas pointed out in Paper 2. This threshold comesabout because the virtuality of the photon can nolonger be neglected when it becomes comparableto its energy. Recently, the total m–N cross-sectionwas reported to increase by 2–3 times if theminimum energy transfer is decreased from 140 to10MeV, based on the parameterisation used inFLUKA [20]. We have confirmed that thisdifference is only 10–15% greater for the200MeV threshold in GEANT4. The aforemen-tioned study also found that the parameterisationused to describe the g–N cross-section in FLUKA[16] (similar to that from Ref. [17] used inGEANT4) overestimates more rigorous theoreti-cal calculations when extrapolated to low energygammas. Consequently, the increase in the muoncross-section with decreasing threshold is notexpected to be as large as mentioned above. Inany case, as pointed out in Paper 2, we expectmany more neutrons to be produced by brems-strahlung (real) photons with low energies inelectromagnetic cascades than by virtual ones inmuon interactions with small energy transfers.
3. Underground neutron fluxes: a case study
The UK Dark Matter Collaboration (UKDMC)has been assessing the feasibility of a xenon-basedtonne-scale dark matter experiment to be installedat the Boulby Underground Laboratory. In thiscontext, initial calculations using FLUKA, re-ported in Paper 3, have so far been performed ofthe muon-induced background in a 250 kg xenontarget. Building on that work we present here acase-study comparison between FLUKA andGEANT4. The calculated neutron fluxes andspectra at the rock/cavern boundary and aftervarious shields are also relevant to other under-ground experiments in different laboratories.
ARTICLE IN PRESS
10-5
10-4
10-3
10-2
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1cos θ
dσ/dΩ
, bar
n/sr
CCuPb
Fig. 7. Differential cross-section of neutron production by190GeV muons for a 10 MeV threshold in neutron energy. Thedata points represent the results of the NA55 experiment. Thethin-line histogram shows the GEANT4 simulation consideringmuon–nucleus interaction only; the thick histogram includes allphysics processes. The dashed line represents the FLUKAresults for the latter case.
H.M. Araujo et al. / Nuclear Instruments and Methods in Physics Research A 545 (2005) 398–411 405
Data Simulation
n-production is severely
underestimated
Araújo, et. al. NIM A, 2005
T.J. Langford WIDG Seminar 2/18/2014���8
Previous Detection Techniques
T.J. Langford WIDG Seminar 2/18/2014���9
Bonner Sphere Arrays
• Passively moderated 3He counters • Deployed in arrays of ~10 detectors
with different diameters • Convert count rates into spectrum
• Relies on response functions • No direct energy observation
T.J. Langford WIDG Seminar 2/18/2014
The geometry and material composition of the 16 counter/moderator combinations was implemented in GEANT4 as realisti-cally as possible. The counter wall was modelled as a 0.5 mm thickstainless steel shell with a 1.5 mm steel ring in the equatorial plane,a 9.25 cm long steel tube at the bottom (1.27 cm diameter, 0.5 mmthick wall) and a 0.95 cm long steel cap on top (1.14 cm diameter,0.5 mm thick wall). The 3He atom density inside the sphericalproportional counter was taken to be 4.25 ! 1019 cm"3. As anexample, Fig.1 shows the geometry of the 7 inch PE spherewith the3He counter in the center as used in the GEANT4 calculations.The density of the PE moderators was equal to 0.95 g cm"3 and thedensity of the leadwas equal to 11.337 g cm"3. The response of eachBonner sphere was calculated for monoenergetic neutrons(1 meVe10 GeV) impinging vertically on top of the sphere (seeFig. 1). The response data obtained from the MC calculations werethen interpolated to generate a response function with 130 energypoints in logarithmic equidistant intervals (i.e. 10 per decade) from1 meV to 10 GeV.
2.2. Bonner spheres spectrometer and unfolding
The data used to test the influence of the various calculatedresponse functions on the deconvoluted neutron fluence ratedistribution were obtained by means of a BSS system consisting of16 spherical 3He proportional counters (Ø 3.3 cm) mostly sensitiveto thermal neutrons. While one of these counters was operatedwithout any moderator, 13 others were surrounded by moderatorsincluding PE shells of various diameters (2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 7,8, 9, 10, 11, 12 and 15 inches, respectively). Additionally, twocounters were surrounded by PE spheres of 9 inch diameter inwhich 0.5 inch and 1 inch thick lead shells were embedded. High-energy secondary neutrons from cosmic radiation, once moderatedand thermalized by these moderators, were finally detected in theproportional counters via the 3He(n,p)3H reaction. This system islocated at the Environmental Research Station (UFS) “Schnee-fernerhaus” (2650 m a.s.l.) on the Zugspitze mountain, Germany(longitude: E 10# 58,90, latitude: N 47# 250), within a measurement
shed constructed on a terrace of the UFS to house the spectrometer.The geomagnetic cutoff corresponded to 4.1 GV for the period ofJanuary and February 2008 (Bütikofer et al., 2007). A more detaileddescription of the geometry of each detector/moderator configu-ration and the measurement location is given in (Leuthold et al.,2007; Rühm et al., 2009a).
The system allows continuous measurements using all 16measurement channels simultaneously. For the present work,a measurement period of one month was chosen (February 2009),and the corresponding average readings of the 16 detectors duringthat period are shown in Fig. 2.
A spectral neutron fluence rate distribution was deduced fromthe experimental data shown in Fig. 2 using the MSANDB unfoldingcode (Matzke, 1987). The MSANDB code is a modified version of theSAND-II code (McElroy et al., 1967) that uses an iterative procedureand requires a priori physical information about the neutronspectrum (i.e. a start/guess spectrum), which represents a firstestimate of the shape of the neutron spectrum at the measurementlocation. Based on this input and the response matrix, MSANDBiteratively adjusts the neutron fluence rate distribution to beconsistent with the count rates measured by the various detectors.The relative uncertainties of the detector counts are used asweighting factors during the iteration process. After a certainnumber of iteration steps, the process is stopped. A guess spectrumincluding three peaks (at 25 meV, 1 MeV, and 100 MeV) and a flatepithermal region, and an iteration number of 300 were used forthe deconvolution process. For details see Simmer et al. (2010).
3. Results and discussion
3.1. Calculated response functions
Fig. 3 shows, as an example, the response functions of the 5, 8,and 12 inch spheres, as obtained using the different transport codesand INC models. It is obvious from the figure that some minordifferences occur in the energy range below about 20 MeV, whichresult from different cross section datasets and from the slightlydifferent geometries used in the calculations. However, significantdifferences are visible for energies above 20MeV. At these energies,the GEANT4 Bertini INC model clearly results in the highest valuesof the response functions, while the GEANT4 Binary INC modelappears to have the same energy dependence, but is consistentlylower. The values obtained with MCNP/LAHET and MCNP/HADRON
Fig. 1. Model of the 7 inch PE sphere with 3He counter as implemented in GEANT4.Tracks of neutrons hitting the sphere from above are also shown.
0
100
200
300
400
500
600
700
0 2 4 6 8 10 12 14 16
h[eta
RtnuoC
1-]
Sphere Diameter [inch]
Fig. 2. Count rates obtained in February 2009 at the UFS “Schneefernerhaus”, for the16 detector/moderator combinations used. X-axis denotes the diameter of the poly-ethylene (PE) spheres surrounding the detectors; 0 inch: detector was not surroundedby any PE; the increased count rates at 9 inch are due to 0.5 inch and 1 inch thick leadshells within the PE moderator.
C. Pioch et al. / Radiation Measurements 45 (2010) 1263e1267 1265
are also lowand roughly similar to those obtainedwith the GEANT4Binary INC model, up to an energy of about 1 GeV. For higherenergies, however, they decrease in contrast to those obtained withboth the GEANT4 Bertini INC and the GEANT4 Binary INC model.This trend is also seen for all other PE spheres (not shown).
For those PE spheres that contain lead shells, differencesbetween the various codes and models are observed in a similarway (Fig. 4) as for the pure PE spheres. The major differencesbecome obvious only above about 20 MeV. Again, use ofthe GEANT4 Bertini INC model provides the highest values for theresponse. In contrast to the results from the pure PE spheres,the MCNP/LAHET results for the spheres with lead are almost ashigh and expectedly do not decrease with increasing neutronenergy above 1 GeV. More specifically, they are very similar to thoseobtained with GEANT4, up to an energy of about 200 MeV. Abovethis energy the GEANT4 Binary INC provides values more thana factor of 2 lower than those obtained with the other codes. Notethat for these two spheres, no HADRON calculations were available,so the response functions of LAHET were used instead to determinefluence and dose rates.
The fact that for the pure PE spheres the GEANT4 Binary INCprovides higher values than the MCNP/LAHET codes while itprovides lower values for those PE spheres that include lead, mayindicate that the nuclear models used in the present work mightnot be equally suitable to model neutron interaction with light andheavy nuclei.
3.2. Neutron fluence rates and neutron ambient dose equivalentrates
While this finding is interesting for itself and asks for a moredetailed analysis of the nuclear models used, it must not necessarilymean that it is important in terms of unfolded neutron spectra, aswell as in terms of dose quantities. To research this inmoredetail, thecount rates shown in Fig. 2 were used and the spectral neutron flu-ence rate distributions were deconvoluted, using the four differentresponse function sets calculated as described above. The resultingfour neutron spectra are shown in Fig. 5 in the lethargy representa-tion. In this representation, equal areas below the curve correspondto equal neutron fluence rates in the considered energy region.
Clearly, there are only minor differences belowa neutron energyof about 1 MeV, while there are some major differences above: Theheight of the cascade peak at 100 MeV is much lower for theGEANT4 Bertini INC than that for GEANT4 Binary INC, MCNP/LAHET, and MCNP/HADRON due to the highest response valuesobtained by the GEANT4 Bertini INC in that energy range (Figs. 3and 4). In order to quantify these differences, the neutron fluencerates deduced from the four neutron spectra integrated over thefour energy regions indicated in Fig. 5 are given in Table 1.
As can be deduced from Table 1, for all codes/models the totalneutron fluence rates obtained do not differ much (less than 4%)from those obtained with the standard MCNP/LAHET responsefunctions. A closer look reveals, however, that the GEANT4ebasedresponse functions result in somewhat larger epithermal neutronfluence rates (12e13%) than those obtained with the MCNP-basedresponse functions. In contrast and more important in terms ofdose (see discussion below), the neutron fluence rate is signifi-cantly lower when based on GEANT4/Bertini INC (about 18%), forneutrons with energies above 20 MeV. This is because GEANT4/Bertini INC resulted in the highest response functions, at theseenergies (see Figs. 3 and 4). Altogether, the neutron spectra
10-2
10-1
100
101
10-9 10-7 10-5 10-3 10-1 101 103
mc[esnopse
R2 ]
Neutron Energy [MeV]
GEANT4 Binary INCGEANT4 Bertini INCMCNP/LAHET
9‘‘ Sphere (1‘‘ lead)
9‘‘ Sphere (0.5‘‘ lead)
Fig. 4. Response functions for those PE sphere that include lead, as obtained using thedifferent neutron transport codes and nuclear models.
0
20
40
60
80
100
120
10−9
10−7
10−5
10−3
10−1
101
103
d(E
mc[)
Ed/2−
h1−]
Neutron Energy [MeV]
MCNP/LAHETMCNP/HADRONGEANT4 Binary INCGEANT4 Bertini INC
ThermalRegion
< 0.4 eV
CascadePeak
> 20 MeV
EpithermalRegion
0.4 eV - 0.1 MeV
EvaporationPeak
0.1 - 20 MeV
.
Fig. 5. Four neutron spectra unfolded from the mean count rates shown in Fig. 2, basedon the response functions calculated with GEANT4 Bertini INC, GEANT4 Binary INC,MCNP/HADRON and MCNP/LAHET, respectively.
Table 1Neutron fluence rates deduced from the neutron spectra of Fig. 5; percent valuesdescribe relative difference to the MCNP/LAHET results.
GEANT4/Bertini(cm!2 h!1)/(%)
GEANT4/Binary(cm!2 h!1)/(%)
MCNP/LAHET(cm!2 h!1)/(%)
MCNP/HADRON((cm!2 h!1)/(%)
Thermal 78.4/3.8 78.7/4.1 75.6/0 75.7/0.1Epithermal 80.4/13.4 79.7/12.4 70.9/0 70.7/!0.4Evaporation 69.1/!2.4 68.6/!3.1 70.8/0 71.1/0.4Cascade 107.4/!18.0 128.6/!1.8 131.0/0 127.2/!2.9Total 335.4/!3.7 355.7/2.1 348.3/0 344.6/!1.0
10-2
10-1
100
101
10-9 10-7 10-5 10-3 10-1 101 103
mc[esnopse
R2]
Neutron Energy [MeV]
GEANT4 Binary INCGEANT4 Bertini INCMCNP/LAHETMCNP/HADRON
12‘‘ Sphere
8‘‘ Sphere
5‘‘ Sphere
Fig. 3. Response functions for the 5, 8, and 12 inch spheres, as obtained using thedifferent neutron transport codes and nuclear models.
C. Pioch et al. / Radiation Measurements 45 (2010) 1263e12671266
Pioch, C. et al. (2010). Rad Meas, 45(10), 1263–1267
erators were 0 cm ! (bare), 8.0 cm ! (1.5 cm thickness),11 cm ! (3.0 cm thickness), 15 cm ! (5.0 cm thickness),and 23 cm ! (9.0 cm thickness), respectively. The schematicdiagram of the measurement system is shown in Fig. 1. Allthe spherical 3He proportional counters with polyethylenemoderators were placed at 1m above the floor. High voltage(þ1;100V) was applied to each detector using high voltagepower supply (ORTEC model 478). Signals from each detec-tor were fed to the pre-amplifier (ORTEC model 142PC) andthe linear amplifier (ORTEC model 571). The output pulsesignals were introduced to a Multi Channel Analyzer(MCA) (ORTEC model 920E) to be converted into the pulseheight spectra. The pulse height spectra obtained by eachdetector were accumulated separately at every fixed time in-terval (1 h) by a personal computer, and summed up above
the cut-off level to discriminate the gamma-ray components.Then the total counts of each 3He counter were obtained bysumming up the accumulated counts above the cut-off levelover a given measurement time at each measuring location.
The response functions were required to convert themeasured counting rates to neutron energy spectrum. Theset of response functions for the energy range from thermalto 1GeV determined by Uwamino et al.8) were adoptedexcept for the bare 3He counter. The response functionof bare 3He counter was evaluated by Nunomiya et al.9)
Figure 2 shows the response functions used in the presentwork. The reliability of the response functions was con-firmed on an experimental basis. The calibration measure-ment of the BS was performed in the monoenergetic neutronfields with energy range of 250 keV to 15MeV at theDynamitron facility of Tohoku University,10) and also inthe thermal neutron field using a 252Cf neutron source andbare 252Cf neutron source in the Facility of RadiationStandards of JAERI (Japan Atomic Energy Research Insti-tute). Throughout the series of the calibration measurements,the differences between the measured and the calculatedvalues were within about 20%.
The cosmic-ray neutron energy spectra were obtainedby unfolding the measured counting rates with the aid ofresponse function. The SAND II code11) was used for thisunfolding, and the cosmic-ray neutron spectrum at sea levelobserved by Goldhagen et al.7) was employed as an initialguess spectrum for unfolding as shown in a solid histogramin Fig. 3. Table 1 shows the analytical conditions forSAND II in this study.
After obtaining neutron spectra by BS, the ambient doseequivalent rate H"(10) and the effective dose rate HE ofthe cosmic-ray at each altitude were estimated on the basisof ICRP Pubication74. In order to estimate the ambient doseequivalent rate H"(10), the neutron fluence to ambient dose
PreAmp.
PreAmp.
Pre Amp.
Pre Amp.
PreAmp.
H.V. Power Supply Linear
Amp.Linear Amp.
Linear Amp.
Linear Amp.
Linear Amp.
Multi Channel Analyzer P.C.
Fig. 1 Schematic diagram of the experimental arrangement of theBonner multi-sphere neutron spectrometer (BS)
0.001
0.01
0.1
1
10
100
1000
1.0E-02 1.0E+00 1.0E+02 1.0E+04 1.0E+06 1.0E+08
Neutron energy (eV)
Res
pons
e (c
ount
sne
utro
ns-1
cm-2
)
Calculated Measured23cm ϕ 23cm ϕ15cm ϕ 15cm ϕ11cm ϕ 11cm ϕ8cm ϕ 8cm ϕbare bare
Fig. 2 Measured and calculated response functions of the Bonner multi-sphere neutron spectrometer (BS)
496 M. KOWATARI et al.
JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY
KOWATARI, M. et al. (2005). J. of Nuc. Sci. and Tech., 42(6), 495–502.
���10
LS Proton Recoil Detectors
CAEN Tools for Discovery
n
Application Note AN2506 Digital Gamma Neutron discrimination with Liquid Scintillators
1
Viareggio
19 November 2012
Introduction In recent years CAEN has developed a complete family of digitizers that consists of several models differing in sampling frequency, resolution, form factor and other features. Besides the use of the digitizers as waveform recorders (oscilloscope mode), CAEN offers the possibility to upload special versions of the FPGA firmware that implement algorithms for the Digital Pulse Processing (DPP); when the digitizer runs in DPP mode, it becomes a new instrument that represents a complete digital replacement of most traditional modules such as Multi Channel Analyzers, QDCs, TDCs, Discriminators and many others. In this application note, we describe the capability of the series x720 (12 bit, 250MSps) to perform neutron-gamma discrimination based upon the digital pulse shape analysis. The development of this FPGA firmware was based upon liquid scintillating detectors of type BC501-A. All detector and neutron/gamma source based tests were performed at the Triangle Universities Nuclear Laboratory (TUNL) on the campus of the Duke University in collaboration with Mohammad Ahmed.
Neutron-Gamma discrimination with liquid scintillators Liquid scintillating detectors are widely used to achieve neutron-gamma discrimination due to their effective Pulse Shape Discrimination (PSD) properties [1]. The light emission of liquid scintillating detectors comprises of a fast decay component, as well as a substantial slow decay component. These components arise from de-excitations of different atomic states in the scintillator. The relative population of these states depends on the energy loss (dE/dx) of the particle. In organic liquid scintillators, these components strongly depend on the energy loss. The gamma rays interact in the scintillator mostly via atomic Compton scattering or pair production mechanisms. The neutrons are detected by scattering them with the protons. These two different processes for gamma-rays and neutrons give rise to significant difference in the slow decay component of the light emission. This difference becomes the basis of pulse shape discrimination in the liquid scintillating detectors [2].
Set-up description
The detector is a 5 inch diameter, 2 inch thick liquid scintillator of type BC501-A. The scintillator is coupled to a Hamamatsu R1250 photo-multiplier tube (PMT). The base is also a Hamamatsu model HTV H6527. The detector is biased to -1430V. The signals from the PMTs are connected to the digitizer inputs via 120 feet low-loss RG8 cables. The detector is biased to -1430V. The signals from the PMTs are connected to the digitizer inputs via 120 feet low-loss RG8 cables. The digitizer is a 4 channel, 12 bit, 250 MS/s DT5720 (Desktop version). The analog bandwidth is about 120 MHz and the input dynamic range is 2 Vpp; the DC offset can be adjusted by means of an internal DAC in the range -2/+2 V. The digitizer runs the preliminary version of the DPP-PSD firmware for the digital charge integration and neutron-gamma discrimination. This principle of operation of the DPP-PSD firmware can be summarized by the following operations that are executed in real time inside the FPGA
x The baseline of the signal is calculated by a programmable length mean filter and subtracted from the input signal; the result is compared with the trigger threshold and, if exceeded, a trigger is issued; it is also possible to use the threshold only to get armed, then wait for the pulse peak and issue the trigger at that time.
x With the trigger, the baseline is frozen, two gates of programmable width are opened and the charge integration starts; the signal fed to the integrator is delayed of a programmable number of samples in order to let the gates to start before the trigger.
x The trigger also initiates the event building: this can include the waveforms (i.e. row samples) of the input, baseline, trigger, gate and other signals, the time stamp of the trigger and the charges calculated within the two gates (short and long).
Fig. 1: The detector used for these measures is a 5”x2” BC501-A liquid scintillator coupled to an Hamamatsu R1250 PMT
Liquid Scintillator
2.5-MeV source. We determined the templates by minimizing thesquared Euclidean distance (L2) of the normalized pulses withineach of the two clusters. We also estimated templates with arobust version of cluster analysis based on an L1 distance metric.In this approach, within each cluster, the median value ratherthan the mean value is computed. The robust and non-robustcluster analysis methods yield similar template estimates.
From the 137Cs gamma-ray source, we determined an electronicrecoil template by a robust signal averaging method. Each baseline-corrected pulse was normalized so that its maximum value was 1.At each time sample, the trimmed mean of all the processed pulseswas computed, and the resulting pulse was divided by its integralvalue. Values of the trimmed mean at each relative time of interestbetween the 0.1 and 0.9 quantiles of the distribution were averaged.For the 2.5-MeV source, we estimated a nuclear recoil template withthe same robust signal averaging method described above. Theestimated nuclear recoil and electronic recoil templates from thecluster analysis agree well with the corresponding robust signalaveraging estimates. Moreover, the estimated nuclear recoil tem-plates determined from start and stop pulses for the 2.5 MeV casewere in very close agreement for the range of amplitudes that weattribute to neutron capture on 6Li.
4.3. Discrimination statistics
The Matusita distances between a normalized pulse of interest, pm,and the template pulses for the electronic recoil pe and nuclear recoilevents pn are
de ¼X
i
ffiffiffiffiffiffiffiffiffiffiffipmðiÞ
p$
ffiffiffiffiffiffiffiffiffiffipeðiÞ
q" #2
ð6Þ
and
dn ¼X
i
ffiffiffiffiffiffiffiffiffiffiffipmðiÞ
p$
ffiffiffiffiffiffiffiffiffiffipnðiÞ
q" #2
ð7Þ
where i is the time increment for the digitized pulse. The normalizedpulses sum to 1. Negative values are set to 0 before taking squareroots in the above equations. Our primary PSD statistic is
logR¼ logdn
de: ð8Þ
For comparison, we also computed a prompt ratio statistic
fp ¼Xp
XTð9Þ
where Xp is the integrated pulse from t¼0 to to and XT is theintegrated pulse over all times. Here, we set to to be the timewhere the nuclear and electronic recoil pulses cross.
For both discrimination statistics, Figs. 8 and 9, we estimate anamplitude dependent discrimination threshold based on events thatproduce logR values less than 0. We then formed a curve in(amplitude, logR) or (amplitude, fp) space. For each method, wesorted the corresponding curve data according to amplitude bins anddetermine the median amplitude and median discrimination statisticwithin each bin. In sequence, we fit a monotonic regression model[52] and then a smoothing spline to each curve. The degrees offreedom of the smoothing spline were determined by cross-valida-tion [53]. We determined a threshold for each particular amplitudeby evaluating the smoothing spline model at that amplitude.
The separation between the logR statistics appears more dramaticthan the separation between the fp statistics for the 137Cs and2.5-MeV sources. Theoretically, we expect that the logR statisticconveys more information because it is based on a 201-bin repre-sentation of the observed pulse whereas the prompt ratio is based ona 2-bin representation of the observed pulse. A careful quantificationof the relative performance of PSD algorithms based on these twostatistics is a topic for further study. One could also form larger binsto smooth out noise before computing a logR statistic for any pulse asdiscussed in Refs. [54,55]. In future experiments, our digital acquisi-tion system will have a higher (10-bit or 12-bit) resolution comparedto the 8-bit resolution of the data shown in this study. This shouldfacilitate refinement of our PSD techniques. In this work, weneglected to account for the energy dependence of the templates.In future work, we may account for this dependence.
5. Summary and conclusions
A liquid scintillator doped with 0.15% 6Li by weight was fabricatedand made into a test cell. The process of making the scintillator does
Fig. 7. Waveform templates for nuclear recoil and electronic recoil eventsdetermined by cluster analysis from calibration data from a 2.5-MeV neutronsource (contaminated by gamma-rays).
Fig. 8. Empirical distribution of logR statistics.
Fig. 9. Empirical distribution of prompt ratio statistics. The width of the prompt timewindow is determined by where the nuclear and electronic recoil templates cross.
B.M. Fisher et al. / Nuclear Instruments and Methods in Physics Research A 646 (2011) 126–134 133
PSD
• N-P scattering to detect deposited energy
• Use pulse shape to separate gammas from neutrons
• All neutron interactions detected, mostly partial-energy depositions
T.J. Langford WIDG Seminar 2/18/2014
131210 1198765432
1MeV
Light Output (MeVee)
Resp
onse
Fun
ctio
n (A
rb. U
nit)
14MeV
1 2 3 4 5 6 7
1
0.01
0.1
1.3.2 The liquid scintillator proton recoil detector
The liquid scintillator proton recoil detector is perhaps the opposite of the
passively moderated 3He counter. Rather than detecting the capture of a thermal-
ized neutron, these detectors function by detecting the recoil proton from a neutron
scatter. With their large hydrogen content, liquid organic scintillators are highly
e↵ective neutron moderators. Depending upon the mass of the recoiling nucleus, a
neutron can transfer any fraction of it’s energy, up to a certain cuto↵ energy.
Er =4A
(1 + A)2(cos2 ✓)En (1.8)
Er|max =4A
(1 + A)2En (1.9)
Er is the energy of the recoiling nucleus, A is the atomic number of the nucleus, ✓
is the angle (in the lab frame) of the recoil, and En is the incident neutron energy.
For hydrogen, the neutron can transfer all of it’s energy to the recoiling proton. For
carbon, the other main element in organic scintillator, the recoil nucleus can receive
a maximum of 0.28 En [12]. Carbon recoils do not produce light in the scintillator,
but manifest as a loss of up to 28% of the total neutron energy.
Liquid scintillator also possesses the important ability to distinguish between
nuclear recoils and electronic recoils using the shape of the resulting pulse of light.
Nuclear recoils excite longer-lived excitations (triplet vs singlet state) in liquid scin-
tillator which lead to a slightly longer pulse of light. Figure 1.19 shows example
traces of typical gamma and neutron interactions in liquid organic scintillator. The
29
���11
Fisher et al. 2011
Ido et al. 2009
C
AE
N
Tool
s fo
r D
isco
very
n
Appl
icat
ion
Not
e AN
2506
Di
gita
l Gam
ma
Neu
tron
disc
rimin
atio
n w
ith Li
quid
Scin
tilla
tors
1
Vi
areg
gio
19 N
ovem
ber 2
012
Intr
oduc
tion
In re
cent
yea
rs C
AEN
has
dev
elop
ed a
com
plet
e fa
mily
of d
igiti
zers
that
con
sists
of s
ever
al m
odel
s di
fferin
g in
sam
plin
g fr
eque
ncy,
re
solu
tion,
for
m f
acto
r an
d ot
her
feat
ures
. Bes
ides
the
use
of
the
digi
tizer
s as
wav
efor
m r
ecor
ders
(os
cillo
scop
e m
ode)
, CAE
N
offe
rs t
he p
ossib
ility
to
uplo
ad s
peci
al v
ersio
ns o
f the
FPG
A fir
mw
are
that
impl
emen
t al
gorit
hms
for
the
Digi
tal P
ulse
Pro
cess
ing
(DPP
); w
hen
the
digi
tizer
run
s in
DPP
mod
e, it
bec
omes
a n
ew in
stru
men
t tha
t rep
rese
nts
a co
mpl
ete
digi
tal r
epla
cem
ent o
f mos
t tr
aditi
onal
mod
ules
suc
h as
Mul
ti Ch
anne
l Ana
lyze
rs, Q
DCs,
TDC
s, D
iscrim
inat
ors
and
man
y ot
hers
. In
this
appl
icat
ion
note
, we
desc
ribe
the
capa
bilit
y of
the
serie
s x7
20 (1
2 bi
t, 25
0MSp
s) to
per
form
neu
tron
-gam
ma
disc
rimin
atio
n ba
sed
upon
the
digi
tal p
ulse
sh
ape
anal
ysis.
The
dev
elop
men
t of t
his F
PGA
firm
war
e w
as b
ased
upo
n liq
uid
scin
tilla
ting
dete
ctor
s of t
ype
BC50
1-A.
Al
l det
ecto
r and
neu
tron
/gam
ma
sour
ce b
ased
test
s w
ere
perf
orm
ed a
t the
Tria
ngle
Uni
vers
ities
Nuc
lear
Lab
orat
ory
(TU
NL)
on
the
cam
pus o
f the
Duk
e U
nive
rsity
in c
olla
bora
tion
with
Moh
amm
ad A
hmed
.
Neu
tron
-Gam
ma
disc
rimin
atio
n w
ith li
quid
scin
tilla
tors
Li
quid
sci
ntill
atin
g de
tect
ors
are
wid
ely
used
to
achi
eve
neut
ron-
gam
ma
disc
rimin
atio
n du
e to
the
ir ef
fect
ive
Pulse
Sha
pe
Disc
rimin
atio
n (P
SD) p
rope
rtie
s [1]
. Th
e lig
ht e
miss
ion
of l
iqui
d sc
intil
latin
g de
tect
ors
com
prise
s of
a f
ast
deca
y co
mpo
nent
, as
wel
l as
a s
ubst
antia
l slo
w d
ecay
co
mpo
nent
. The
se c
ompo
nent
s ar
ise f
rom
de-
exci
tatio
ns o
f di
ffere
nt a
tom
ic s
tate
s in
the
sci
ntill
ator
. The
rel
ativ
e po
pula
tion
of
thes
e st
ates
dep
ends
on
the
ener
gy lo
ss (d
E/dx
) of t
he p
artic
le. I
n or
gani
c liq
uid
scin
tilla
tors
, the
se c
ompo
nent
s str
ongl
y de
pend
on
the
ener
gy lo
ss.
The
gam
ma
rays
inte
ract
in th
e sc
intil
lato
r mos
tly v
ia a
tom
ic C
ompt
on s
catt
erin
g or
pai
r pro
duct
ion
mec
hani
sms.
The
neu
tron
s ar
e de
tect
ed b
y sc
atte
ring
them
with
the
prot
ons.
The
se tw
o di
ffere
nt p
roce
sses
for
gam
ma-
rays
and
neu
tron
s gi
ve r
ise to
sig
nific
ant
diffe
renc
e in
the
slo
w d
ecay
com
pone
nt o
f the
ligh
t em
issio
n. T
his
diffe
renc
e be
com
es t
he b
asis
of p
ulse
sha
pe d
iscrim
inat
ion
in
the
liqui
d sc
intil
latin
g de
tect
ors [
2].
Set-
up d
escr
iptio
n
The
dete
ctor
is a
5 in
ch d
iam
eter
, 2 in
ch th
ick
liqui
d sc
intil
lato
r of t
ype
BC50
1-A.
The
sci
ntill
ator
is
coup
led
to a
Ham
amat
su R
1250
pho
to-
mul
tiplie
r tu
be (
PMT)
. Th
e ba
se i
s al
so a
Ham
amat
su m
odel
HTV
H6
527.
The
det
ecto
r is b
iase
d to
-143
0V.
The
signa
ls fr
om t
he P
MTs
are
con
nect
ed t
o th
e di
gitiz
er in
puts
via
12
0 fe
et lo
w-lo
ss R
G8 c
able
s.
The
dete
ctor
is
bias
ed t
o -1
430V
. Th
e sig
nals
from
the
PM
Ts a
re
conn
ecte
d to
the
digi
tizer
inpu
ts v
ia 1
20 fe
et lo
w-lo
ss R
G8 c
able
s.
The
digi
tizer
is
a 4
chan
nel,
12 b
it, 2
50 M
S/s
DT57
20 (
Desk
top
vers
ion)
. Th
e an
alog
ban
dwid
th i
s ab
out
120
MHz
and
the
inp
ut
dyna
mic
rang
e is
2 Vp
p; th
e DC
offs
et c
an b
e ad
just
ed b
y m
eans
of a
n in
tern
al D
AC in
the
rang
e -2
/+2
V.
The
digi
tizer
runs
the
prel
imin
ary
vers
ion
of th
e DP
P-PS
D fir
mw
are
for
the
digi
tal c
harg
e in
tegr
atio
n an
d ne
utro
n-ga
mm
a di
scrim
inat
ion.
Thi
s pr
inci
ple
of o
pera
tion
of th
e DP
P-PS
D fir
mw
are
can
be su
mm
arize
d by
th
e fo
llow
ing
oper
atio
ns t
hat
are
exec
uted
in
real
tim
e in
side
the
FPGA
x Th
e ba
selin
e of
the
sig
nal i
s ca
lcul
ated
by
a pr
ogra
mm
able
leng
th m
ean
filte
r an
d su
btra
cted
from
the
inpu
t sig
nal;
the
resu
lt is
com
pare
d w
ith th
e tr
igge
r thr
esho
ld a
nd, i
f exc
eede
d, a
trig
ger i
s iss
ued;
it is
also
pos
sible
to u
se th
e th
resh
old
only
to g
et a
rmed
, the
n w
ait f
or th
e pu
lse p
eak
and
issue
the
trig
ger a
t tha
t tim
e.
x W
ith th
e tr
igge
r, th
e ba
selin
e is
froz
en, t
wo
gate
s of
pro
gram
mab
le w
idth
are
ope
ned
and
the
char
ge in
tegr
atio
n st
arts
; th
e sig
nal f
ed to
the
inte
grat
or is
del
ayed
of a
pro
gram
mab
le n
umbe
r of s
ampl
es in
ord
er to
let t
he g
ates
to s
tart
bef
ore
the
trig
ger.
x Th
e tr
igge
r al
so in
itiat
es t
he e
vent
bui
ldin
g: t
his
can
incl
ude
the
wav
efor
ms
(i.e.
row
sam
ples
) of
the
inpu
t, ba
selin
e,
trig
ger,
gate
and
oth
er s
igna
ls, th
e tim
e st
amp
of th
e tr
igge
r and
the
char
ges
calc
ulat
ed w
ithin
the
two
gate
s (s
hort
and
lo
ng).
Fig.
1: T
he detec
tor u
sed for t
hese m
easu
res is a
5”x2”
BC5
01-A
liq
uid
scin
tilla
tor c
oupl
ed to
an
Ham
amat
su R
1250
PM
T
Bonner Spheres LS Proton Recoil
• Only measure thermal neutron capture
• Very difficult to calibrate response functions
• No direct energy observation
• Detect energy from every interaction
• PID from pulse shape • Most events are partial energy
deposition, bad for spectroscopy
Vs
T.J. Langford WIDG Seminar 2/18/2014
erators were 0 cm ! (bare), 8.0 cm ! (1.5 cm thickness),11 cm ! (3.0 cm thickness), 15 cm ! (5.0 cm thickness),and 23 cm ! (9.0 cm thickness), respectively. The schematicdiagram of the measurement system is shown in Fig. 1. Allthe spherical 3He proportional counters with polyethylenemoderators were placed at 1m above the floor. High voltage(þ1;100V) was applied to each detector using high voltagepower supply (ORTEC model 478). Signals from each detec-tor were fed to the pre-amplifier (ORTEC model 142PC) andthe linear amplifier (ORTEC model 571). The output pulsesignals were introduced to a Multi Channel Analyzer(MCA) (ORTEC model 920E) to be converted into the pulseheight spectra. The pulse height spectra obtained by eachdetector were accumulated separately at every fixed time in-terval (1 h) by a personal computer, and summed up above
the cut-off level to discriminate the gamma-ray components.Then the total counts of each 3He counter were obtained bysumming up the accumulated counts above the cut-off levelover a given measurement time at each measuring location.
The response functions were required to convert themeasured counting rates to neutron energy spectrum. Theset of response functions for the energy range from thermalto 1GeV determined by Uwamino et al.8) were adoptedexcept for the bare 3He counter. The response functionof bare 3He counter was evaluated by Nunomiya et al.9)
Figure 2 shows the response functions used in the presentwork. The reliability of the response functions was con-firmed on an experimental basis. The calibration measure-ment of the BS was performed in the monoenergetic neutronfields with energy range of 250 keV to 15MeV at theDynamitron facility of Tohoku University,10) and also inthe thermal neutron field using a 252Cf neutron source andbare 252Cf neutron source in the Facility of RadiationStandards of JAERI (Japan Atomic Energy Research Insti-tute). Throughout the series of the calibration measurements,the differences between the measured and the calculatedvalues were within about 20%.
The cosmic-ray neutron energy spectra were obtainedby unfolding the measured counting rates with the aid ofresponse function. The SAND II code11) was used for thisunfolding, and the cosmic-ray neutron spectrum at sea levelobserved by Goldhagen et al.7) was employed as an initialguess spectrum for unfolding as shown in a solid histogramin Fig. 3. Table 1 shows the analytical conditions forSAND II in this study.
After obtaining neutron spectra by BS, the ambient doseequivalent rate H"(10) and the effective dose rate HE ofthe cosmic-ray at each altitude were estimated on the basisof ICRP Pubication74. In order to estimate the ambient doseequivalent rate H"(10), the neutron fluence to ambient dose
PreAmp.
PreAmp.
Pre Amp.
Pre Amp.
PreAmp.
H.V. Power Supply Linear
Amp.Linear Amp.
Linear Amp.
Linear Amp.
Linear Amp.
Multi Channel Analyzer P.C.
Fig. 1 Schematic diagram of the experimental arrangement of theBonner multi-sphere neutron spectrometer (BS)
0.001
0.01
0.1
1
10
100
1000
1.0E-02 1.0E+00 1.0E+02 1.0E+04 1.0E+06 1.0E+08
Neutron energy (eV)
Res
pons
e (c
ount
sne
utro
ns-1
cm-2
)
Calculated Measured23cm ϕ 23cm ϕ15cm ϕ 15cm ϕ11cm ϕ 11cm ϕ8cm ϕ 8cm ϕbare bare
Fig. 2 Measured and calculated response functions of the Bonner multi-sphere neutron spectrometer (BS)
496 M. KOWATARI et al.
JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY
���12
PMT PM
T
Capture Gated Spectroscopy
pt
p
500
400
300
200
Chan
nel
155154153152151150149148x10
3
pp
4000
3950
3900
3850
Chan
nel
36.98x10336.9636.9436.9236.9036.8836.86Samples
4000
3950
3900
3850
Chan
nel
36.98x10336.9636.9436.9236.9036.8836.86Samples
4000
3950
3900
3850
Chan
nel
36.98x10336.9636.9436.9236.9036.8836.86Samples
Combine features from BS and PR detectors: 1.Demand capture
- Full energy deposition 2.Directly measure deposited energy
T.J. Langford WIDG Seminar 2/18/2014
n Scintillator
3He
���13
The FaNS Detectors
T.J. Langford WIDG Seminar 2/18/2014���14
The FaNS Detectors• Arrays of plastic scintillator segments and 3He
proportional counters
• Segmentation improved energy reconstruction
• Use Capture-gated Spectroscopy for particle identification and energy information
• Calibrated at NIST with Cf-252, DD, and DT neutrons
• FaNS-1: Rapidly deployed as proof-of-principle
• FaNS-2: Full fledged detector
T.J. Langford WIDG Seminar 2/18/2014���15
30
25
20
15
10
5
0Li
ght
Units
(M
eVee
)302520151050
Energy (MeV)
Linear Response Proton Response
Energy Resolution Through Segmentation
• Plastic scintillator has a non-linear light response to heavy charged particles
• To achieve better energy resolution, we segment the detector to reconstruct each scatter separately
• Segment size is chosen to match the mean free path of target neutrons
We want to detect each scatter separately, convert light into energy, and then sum
WIDG Seminar 2/18/2014T.J. Langford ���16
CGS with FaNS
Proton Recoils
3He Neutron Capture�t
4000
3000
2000
1000
0
-1000
Chan
nel (
arb)
-15 -10 -5 0 5 10 15Time (us)
Block8, PMT1 Block8, PMT2 Block12, PMT1 Block12, PMT2 He3
• Trigger on the 3He capture • Record scintillator signals that
occur within a fixed time before the capture
• Negative coincidences must be random
T.J. Langford WIDG Seminar 2/18/2014���17
Time Separation Spectrum
T.J. Langford WIDG Seminar 2/18/2014���18
Random Coincidences
Real Coincidences
Time Separation Spectrum
T.J. Langford WIDG Seminar 2/18/2014���19
FaNS-1 Summary
• 18liters of PS in six segments • Six He-3 proportional counters • Calibrated at NIST • Measured surface spectrum to 150MeV • Installed at Kimballton Lab • Measured the neutron spectrum and flux at KURF
700
600
500
400
300
200
100
0
Coun
ts (
arb)
543210Energy (MeV)
Data Monte Carlo
600
500
400
300
200
100
0
Coun
ts (
arb)
151050Pulse Ht (MeV)
Data Monte Carlo
���20T.J. Langford WIDG Seminar 2/18/2014
DD
DT
FaNS-1Surface Neutrons
10-7
10-6
10-5
10-4
10-3
10-2
10-1
Coun
ts /
MeV
/s
5 6 7 8 91
2 3 4 5 6 7 8 910
2 3 4 5 6 7 8 9100
2 3
Energy (MeV)
BG Subtracted Scaled MCNP
T.J. Langford WIDG Seminar 2/18/2014
�(En > 1 MeV ) = (4.0± 1.0)⇥ 10�3n/cm2/s
���21
FaNS-1 at KURF - 1450 mwe
3
4567
1
2
3
4567
10
2
3
4567
100
Coun
ts/M
eV
8642Neutron Energy (MeV)
Real Coincidences Random Coincidences
1.0
0.8
0.6
0.4
0.2
0.0
Rise
Tim
e (u
s)
543210Pulse Ht (MeV)
↵
µD
n
�
• 3.74x106 s dataset • 1.67x105 He-3 triggers • 384 counts pass all cuts • 89 remain after BG
subtraction
WIDG Seminar 2/18/2014T.J. Langford
30
20
10
0
Coun
ts
-200 -100 0 100 200Time Separation (µs)
���22
FaNS-1 at KURF - 1450mwe
8910-7
2
3
4
5678910-6
2
3
4
5678910-5
Coun
ts/M
eV/s
8642Neutron Energy (MeV)
BG Subtracted
T.J. Langford WIDG Seminar 2/18/2014
15
events/keV-kg-year, is precisely what we have found inour simulation.
Depth (km.w.e.)1 2 3 4 5 6
)-1 y
ear
-1 k
g-1
Even
ts (k
eV
-710
-610
-510
-410
-310
-210
-110
12510
2610
2710
2810
Hal
f Life
(yea
rs)
Majorana
Granularity, PSD and Segmentation
Target sensitivity
Without neutron veto
With neutron veto (99%)
WIP
P
Gra
n Sa
sso
Sudb
ury
FIG. 25: The Depth-Sensitivity-Relation (DSR) derived fora Majorana-like experiment showing, specifically, the resultsfrom this work assuming the detector is operated at a depthequivalent to the Gran Sasso Laboratory. The raw event ratein the energy region of interest of 0.026 events/keV-kg-yearcan be reduced by a factor of 7.4 by exploiting the detectorgranularity, pulse-shape discrimination (PSD), and detectorsegmentation. The upper curve displays the background sim-ulated in the case that no active neutron veto is present andthe lower curve indicates the reduction that would ensue if anactive neutron veto were present that is 99% efficient.
The results of our simulations can be used to derivethe DSR for Majorana as shown in Fig. 25. The neu-tron induced background can be reduced by about a fac-tor of 7.4 in Majorana owing to the use of crystal-to-crystal coincidences and the use of pulse-shape discrim-ination and segmentation. Nonetheless, to achieve thetarget sensitivity of next generation double-beta decayexperiments, 0.00025 events/keV-kg-year correspondingto the background level required to reach sensitivity tothe atmospheric mass scale of 45 meV Majorana neutrinomass, the muon-induced background must be reduced byroughly another factor of 100. This can be achieved onlyby operating such a detector at depths in excess of 5km.w.e., otherwise an active neutron veto would need tobe implemented with an efficiency in excess of 99%.
D. (α,n) Background
Once the depth requirement is satisfied, a proper shieldagainst (α,n) neutrons from the environment becomesnecessary. We use the standard rock and the measuredneutron flux (3.78×10−6cm−2s−1 [68, 69] ) at Gran Sassoassuming that all underground labs have the same orderof neutron flux to establish the shielding requirement for(α, n) neutrons. This flux corresponds to an average ofabout 2.63 ppm 238U and 0.74 ppm 232Th activity inGran Sasso rock and 1.05 ppm 238U and 0.67 ppm 232Thactivity in Gran Sasso concrete [70]. The neutron energy
spectrum depending on the rock composition is shown inFig. 26. As can be seen, the total neutron flux is aboutthree orders of magnitude higher than that of neutronsfrom the rock due to muon-induced processes, but theenergy spectrum is much softer.
Neutron Energy (MeV)0 1 2 3 4 5 6 7 8 9 10
)-1 M
eV-1
s-2
Neu
tron
s (c
m
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
0.22-610×
Standard Rock
Gran Sasso Rock
FIG. 26: The neutron energy spectrum arising from (α,n)reactions due to radioactivity in the rock. We predict a harderenergy spectrum in Gran Sasso rock relative to standard rockowing to the presence of carbon and magnesium.
To demonstrate the neutron flux and energy spectrumat different boundaries we show the rock/cavern neutronflux and energy spectrum with a shielding for Majoranadescribed earlier in Fig. 27 for the depth of Gran Sassoas an example.
Energy (MeV)-110 1 10 210 310
)-1 M
eV-1
y-2
Neu
tron
s (c
m
-1210-1110
-1010
-910
-810
-710
-610
-510
-410
-310
-210
-1101
10
210
310:Rock/Cavern boundaryµ
:After poly shieldingµ
:After lead + copper shieldingµ
,n): Rock/Cavern boundaryα(
,n): After poly + lead + copper shieldingα(
FIG. 27: The energy spectrum for fast neutrons producedby (α, n) reactions in the rock compared to those induced bymuon interactions in the rock with and without shielding. Thelower energy neutrons (< 10 MeV) are quickly absorbed usingpolyethylene shielding, however, the high energy portion ofthe muon-induced neutron flux persists. The addition of leadshielding adjacent to a detector can also create an additionalsource of muon-induced neutrons.
Note that the (α, n) neutrons from the rock are quicklyattenuated to the level of the muon-induced neutrons be-
(↵, n)
���23
�(En > 1.4 MeV) = (1.6± 0.4)⇥ 10�6 n/cm2/s
FaNS-1 Conclusions• Demonstrates the power of CGS
• Detect peaks rather than edges
• Measured the surface fast neutron spectrum and flux from 1 to 150 MeV
• Deployed FaNS-1 at KURF, where an (alpha, n)-like spectrum and flux were successfully measured
• Influenced design of FaNS-2
• Focus on muon-induced neutrons (En > 10 MeV)
• Operate at a shallow location to measure flux and spectrum
T.J. Langford WIDG Seminar 2/18/2014���24
FaNS-2
T.J. Langford WIDG Seminar 2/18/2014���25
T.J. Langford WIDG Seminar 2/18/2014
FaNS-2
���26
T.J. Langford WIDG Seminar 2/18/2014
FaNS-221 3He Neutron
Detectors
9cm X 9cm X 56cm Plastic Scintillator Bars
���27
FaNS-2• The “Purpose-built” Detector
• Improvement over FaNS-1 in almost every way:
• Volume: 15 liters → 72 liters
• DAQ: twice as fast, 8ch → 56ch
• PMTs: better linearity and characterization
• Built at UMD, commissioned at NIST in December 2012
• FaNS-2 was operated in a low scatter room measuring source and ambient neutrons
• Reduce backscattering neutrons compared to FaNS-1
• Effectively no shielding of ambient neutrons from cosmic rays
T.J. Langford WIDG Seminar 2/18/2014���28
Cf-252 Source Calibration2.5
2.0
1.5
1.0
0.5
0.0
Coun
t ra
te (
n/s)
> 2
MeV
25x10-320151050Solid Angle Subtended
MCNP Corrected Data
Dataa =0.04 ± 0.03b =77.0 ± 2.5
MCNPa =0.035 ± 0.007b =80.6 ± 0.8
• Seven different source positions were measured - (85 cm - 238 cm above FaNS-2) and two runs without the source
• Each position was modeled in MCNP, and the same cuts were applied to data and simulation
• The slope of the fit-line is extracted for the efficiency
T.J. Langford WIDG Seminar 2/18/2014
Cf
✏ =slope
source activity
= (3.6± 0.15)%
���29
T.J. Langford WIDG Seminar 2/18/2014
Target AcceleratingPotential
Drift RegionHV
Feedthrough
Neutron Generator Data
D +D ! 3He+ n (2.5 MeV )D + T ! 4He+ n (14.1 MeV )
2500
2000
1500
1000
500
0
Coun
ts
43210Energy (MeV)
Data MCNPDD
600
400
200
0
Coun
ts
20151050Energy (MeV)
Data MCNPDT
���30
Cosmogenic Neutrons at NISTn
nn
Isotropic neutron field
Gordon et al. Surface Neutron Spectrum
• Determine the response of FaNS-2 to an assumed cosmogenic neutron spectrum • Use the response to convert measured neutrons into incident flux
✏ = 87± 13 n/(fluence)
T.J. Langford WIDG Seminar 2/18/2014���31
• Operated periodically over the course of 6 months
• 4x106 triggers were collected
• Post BG subtraction,1.7x106 events remain
Cosmogenic Neutrons at NIST
T.J. Langford WIDG Seminar 2/18/2014
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
Coun
ts /
MeV
/s
100 101 102 103 104
Energy (MeV)
Real+Random Random Only
2.5x10-3
2.0
1.5
1.0
0.5
0.0
Coun
ts/s
6004002000-200Time Separation (µs)
���32
1050
1040
1030
1020
1010
1000
Barometric Pressure (m
b)
3/16/13 3/21/13 3/26/13 3/31/13 4/5/13 4/10/13Date
0.70
0.65
0.60
0.55
0.50
0.45
0.40
FaNS
-2 C
ount
Rat
e (n
/s)
Barometric Variation
���33
0.70
0.68
0.66
0.64
0.62
0.60
0.58
0.56
0.54
FaNS
-2 B
GSub
Rat
e (1
/s)
3/16/13 3/21/13 3/26/13 3/31/13 4/5/13 4/10/13Date
105
100
95
90
85
Newark Neutron M
onitor Rate (1/s)
NMDB FaNS-2
100
90
80
70
60
Neu
tron
Mon
itor R
ate
(n/s
)
3/16/13 3/21/13 3/26/13 3/31/13 4/5/13 4/10/13Date
1050
1040
1030
1020
1010
1000
Barometric Pressure (m
b)
http://www.nmdb.eu/T.J. Langford WIDG Seminar 2/18/2014
• Neutron monitoring stations to study solar activity are spread across the globe
• Data freely available!
• Can compare pressure corrected and uncorrected event rates
94
92
90
88
86
Newark Neutron M
onitor Rate (1/s)
3/16/13 3/21/13 3/26/13 3/31/13 4/5/13 4/10/13Date
0.66
0.64
0.62
0.60
0.58
0.56
FaNS
-2 B
GSub
Rat
e (1
/s)
NMDB FaNS-2
T.J. Langford WIDG Seminar 2/18/2014
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
Coun
ts /
MeV
/s
100 101 102 103 104
Energy (MeV)
Data MCNP
�(n) =(1.753± 0.003)⇥ 106 n
4.842⇥ 106 s⇥ (87± 13) n/(n/cm2)
�(n) = (4.16± 0.6)⇥ 10�3 n/cm2/s
Flux above 2 MeV
Cosmogenic Neutrons at NIST
���34
Cosmogenic Neutrons at NIST
T.J. Langford WIDG Seminar 2/18/2014
2.5x10-3
2.0
1.5
1.0
0.5
0.0
E x
dφ/d
E (n
/s)
100 101 102 103 104
Energy (MeV)
Data MCNP
Directly observe features at 1.5 and 4 MeV that
BBS cannot resolve
���35
CGS Measurement at LNGS
Direct measurement of the atmospheric neutron flux in the energyrange 10–500 MeV
Antonio Bonardi a,b,c,*, Marco Aglietta b,c, Gianmarco Bruno d,e, Walter Fulgione b,c,Ana Amelia Bergamini Machado e
a Università di Torino: via Giuria 1, 10125 Torino, Italyb INFN Torino: via Giuria 1, 10125 Torino, Italyc IFSI Torino: Corso Fiume 4, 10133 Torino, Italyd Università de l’Aquila: Via Vetoio Località Coppito, 67100 L’Aquila, Italye INFN-Laboratori Gran Sasso: S.S. 17 BIS km 18.910, 67010 Assergi L’Aquila, Italy
a r t i c l e i n f o
Article history:Received 24 May 2010Received in revised form 27 July 2010Accepted 28 July 2010Available online 5 August 2010
Keywords:NeutronsFluxMeasurement
a b s t r a c t
The results of a direct measurement of the atmospheric neutron flux in the energy range 10–500 MeVperformed at 42!2501100 N, 13!310200 E, rigidity cutoff 6.3 GV, altitude 970 m a.s.l. (LNGS external site)on November 2008, during minimum solar activity, are reported.
The detector consists of a 1.5 ! 1 ! 1 m3 stainless steel tank filled with 1.2 tons of 0.1% Gd doped liquidscintillator, monitored by three photomultipliers and surrounded by a 4p active muon veto. The mea-surement is performed by observing events formed by two un-vetoed pulses inside a temporal window95 ls long: the first one due to a recoiling proton scattered by a neutron, the second one due to the neu-tron capture (n,Gd).
The resulting atmospheric neutron fluxes are:
UðE > 10 MeVÞ ¼ 47% 5 neutrons s&1 m&2;
UðE > 20 MeVÞ ¼ 42% 4 neutrons s&1 m&2:
" 2010 Elsevier B.V. All rights reserved.
1. Introduction
All the neutrons produced by the interactions of galactic and so-lar cosmic rays in the atmosphere are called ‘‘atmospheric neu-trons”. They are generated in hadronic and electromagnetic airshowers by spallation and evaporation processes on Nitrogen andOxygen nuclei.
Neutrons can be dangerous for electronic devices and humanhealth at aviation altitude where they contribute to about half ofthe dose equivalent of the secondary cosmic radiation [1–3],whereas at sea level they are a significant background source forseveral types of surface detectors. In particular our attention hereis focused on studying the atmospheric neutron background for aliquid scintillator antineutrino surface detector.
Since the famous Cowan and Reines experiment [4], nuclearpower plants have been used for studying neutrino properties
and nowadays neutrino oscillation experimental observations arereactivating this experimental approach.
In principle, it is possible to turn the tide, i.e. to use antineutrinodetection for monitoring the activity of nuclear power plants:this may be very useful in particular for detecting illicit or suspi-cious uses of these facilities [5]. The International Atomic EnergyAgency is in the process of developing new technologies to monitornuclear activity on power plants, aiming for non-proliferationsafeguards.
The advantages of using !me for reactor safeguards and monitoringinclude the availability of real time information on the status of thecore, less intrusiveness, and simplified operations from the stand-point of both the reactor operator and the safeguards agency [5].
Such a !me detector has to be small ('1 m3) [5] and not overbur-dened, due to cost and logistical reasons: that means a very poorshielding factor for cosmic radiation.
Up to now, !me detectors are designed to look for Inverse Beta De-cay (IBD) !me þ p) eþ þ n and their active mass is monitored by pho-tomultiplier tubes (PMTs). In case of IBD the prompt e+ signal isfollowed by a delayed one due to neutron capture: the double signa-ture allows the disentangling of !me signals from random background.
0927-6505/$ - see front matter " 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.astropartphys.2010.07.004
Corresponding author at: Università di Torino, Via Giuria 1, 10125 Torino, Italy.Tel.: +39 3497942509.
E-mail address: [email protected] (A. Bonardi).
Astroparticle Physics 34 (2010) 225–229
Contents lists available at ScienceDirect
Astroparticle Physics
journal homepage: www.elsevier .com/ locate/ast ropart
(MeV)visE10 210
)−
1flu
x (H
z M
eV
−310
−210
−110
1
• 1.2 m3 liquid scintillator doped with Gd, operated for 5 days
• Collected ~6x103 neutron events
• Measured flux above two thresholds (10 and 20 MeV) with 10% uncertainties
T.J. Langford WIDG Seminar 2/18/2014
Conversion based on altitude, Geomag. cut-off, and solar activity to estimate location dependence
of measurements
���36
Gordon et al. 2004
NIST vs Gran Sasso National Lab
Table 7.14: A comparison between the LNGS and FaNS-2 measurements of theneutron flux above 10 MeV and 20 MeV.
Energy Range LNGS Flux Corrected LNGS FaNS-2 Fluxn/cm2/s n/cm2/s n/cm2/s
En > 10 MeV (4.7±0.5)⇥ 10�3 (2.9±0.3)⇥ 10�3 (3.05± 0.4)⇥ 10�3
En > 20 MeV (4.2±0.4)⇥ 10�3 (2.6±0.3)⇥ 10�3 (2.75± 0.4)⇥ 10�3
During the course of six months, the detector was stable and operated ex-
tremely well. FaNS-2 has demonstrated the ability to measure very high energy
neutron events while still having sensitivity to low energy neutrons with good en-
ergy resolution. The comparison between FaNS-2 data and the MCNP simulation
is now discussed, along with a comparison between the two FaNS detectors.
7.8.1 Comparison to Monte Carlo
Though there are many fluctuations in measurements of the total flux of
cosmic-ray induced neutrons, the relative shape of the energy spectrum is sim-
ilar across measurements. Thus, the Monte Carlo has been used to determine
the weighted response (in neutron detected per neutron fluence) for FaNS-2 in the
cosmic-ray induced neutron field. This response has then been applied to the de-
tected neutron rate to obtain a measurement of the incident neutron flux.
Between 1 and 200 MeV the data and Monte Carlo spectra agree very well,
including features at 1 MeV and 3 MeV that appear in both spectra. These fea-
tures are part of the simulated spectrum because of calculations of resonances in
the atmosphere. However, Bonner sphere measurements lack the energy resolution
required to observe them. FaNS-2 has been able to directly observe these features.
241
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
Coun
ts /
MeV
/s
100 101 102 103 104
Energy (MeV)
Data MCNP
(MeV)visE10 210
)−
1flu
x (H
z M
eV
−310
−210
−110
1CGS Measurement
at LNGS
T.J. Langford WIDG Seminar 2/18/2014
To compare between LNGS and NIST, a
conversion factor (x1.64) based on location, and
solar cycle has been applied
Bonardi et al. Astroparticle Phys 34 (2010) 225–229
���37
FaNS Outlook• Both FaNS-1 and FaNS-2 are operating at NIST
• FaNS-1 is characterizing fast neutron backgrounds at reactors for PROSPECT, a new neutrino oscillation experiment that will operate at sea-level
• FaNS-2 is measuring the muon-induced neutron spectrum at a depth of 20 m.w.e.
• Work is underway to better understand the deviation between data and MCNP for En > 200 MeV
• The FaNS program has definitively demonstrated the benefit of CGS for neutron detection
• The FaNS detectors are improving NIST’s functionality for precision neutron spectroscopy
T.J. Langford WIDG Seminar 2/18/2014���38
Conclusions• The surface and underground fast neutron backgrounds pose
serious threats to the feasibility of current and future low-background experiments
• Current detection techniques lack the ability to fully characterize the spectrum and flux of these backgrounds
• Two novel detectors based on Capture Gated spectroscopy have been developed and calibrated by the UMD/NIST collaboration
• FaNS-1 successfully performed a measurement of the surface fast neutron spectrum before being deployed at KURF
• FaNS-1 characterized the fast neutron background at KURF demonstrating that the spectrum is (alpha,n)-like
• FaNS-2 greatly improved on the measurement of the surface fast neutron spectrum, and made a definitive measurement of the flux
T.J. Langford WIDG Seminar 2/18/2014���39