applications of thermal neutron - istituto nazionale di...

Giorgia Albani - Universit Milano Bicocca

Applications of thermal neutron detectors alternative to 3He in

neutron spallation sources

Giorgia AlbaniXXIX PhD StudentUniversit Milano BicoccaSupervisor: prof. Giuseppe Gorini

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Outline

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THE PROBLEM OF 3He SHORTAGE

GEM-BASED NEUTRON DETECTORS Prototypes Tests

THE BAND-GEM DETECTOR Concept design Applications

Small Angle Neutron Scattering Neutron Diffraction

ESS COLLABORATION

RESULTS

CONCLUSIONS

FUTURE PERSPECTIVES


Neutron spallation sources

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3He shortage

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3He shortage

17/09/14 Giorgia Albani - Annual Seminar year 5

3He was produced by nuclear weapons program

3H (t1/2 = 12,3 y) 3He + e- + antineutrino

High efficiency thermal neutron detectors shall be developed in order to replace 3He detectors in the future spallation sources (ESS).

3He detectors are limited in spatial resolution and counting rate capability

By far the 3He is the most used converter for thermal neutron detection: High neutron absorption cross section: n + 3He ! p + 3H (5330 b) Non radioactive Non toxic

I n sono neutri e per essere rivelati devono essere convertiti in pcelle cariche tramite reazioni nucleari. Il convertitore da sempre pi utilizzato per neutroni termiciquindi da sempre stato utilizzato per costruire tubi a 3He che convertono n in protoni. La disponibilit era imponente dopo la guerra fredda. Ce n poco in natura era stato prodotto nel programma degli armamenti nuceari dal decadimento del trizio ma il suo tempo di dimezzamento di soli 12 anni dopo lo stop degli armamenti la scorta si ridotta e il prezzo dell 3He diventato proibitivo. Nuovi rivelatori ad alta efficienza

3He tubes

Giorgia Albani - Universit Milano Bicocca 5

My PhD project aims toDevelop a complete optimization of a Boron Advanced Neutron Detector based on Gas Electron Multiplier (BAND-GEM) technology on the LOKI instrument for Small Angle Neutron Scattering (SANS) measurements at

the European Spallation Source (ESS)

ESS will provide around 30 times brighter neutron beams than existing facilities today. high beam intensity and long pulses (3 ms) Repetition rate 14 Hz


GEM detectors Neutrons are converted with a nuclear reaction in the converter cathode into charged particles, that reach the drift region and ionize the gas. Electrons from primary ionization enter in the intense dipole field inside the GEM foi ls holes (moltiplication region) and cause secondary ionization. The signal is inducted on the padded anode by the movement of the electrons in the induction region towards the anode.

Advantages: High rate capability (MHz/mm2) Negligible discharge probability Large areas ( 1m2) Adaptable readout structure


Detecting neutrons with GEM

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G. Croci, G.Albani, C. Cazzaniga, E. Perelli Cippo, M. Tardocchi and G.Gorini, Diffraction measurements with a boron-based GEM neutron detector, EPL 107 (2014) 12001.


The new idea: a 3D cathode

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Increase the efficiency

Enhance the thickness of the conversion layer but allow the conversion products to reach the gas

3D cathode before the GEM foils

Properly positioning the detector in the beam the lamella system allows to significantly increase the thickness of the borated material

crossed by neutron


Scheme and principle of operation

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Application of GEM-based detectors to neutron diffraction

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Neutron diffraction @ ISIS

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bGEM borated cathode

INES (ISIS) sample pos.

Incident neutron beam

Transmitted neutron beam

GEM position 128 8x8 mm2 pads

Interface with ISIS-DAE: Time of Flight measurement performed using standard ISIS TOF DAE

G. Croci, G.Albani, C. Cazzaniga, E. Perelli Cippo, M. Tardocchi and G.Gorini, Diffraction measurements with a boron-based GEM neutron detector, EPL 107 (2014) 12001.


Improvements in GEM-based detectors

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Cathode with enriched 10B instead of natural boron

3D cathode instead of a planar cathode

Focussing

Collimator

EFFICIENCY S/B RATIO


Efficiency comparison with 3He

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First test (natural boron)

3He counted 25 times the all GEM

Second test (enriched boron) 3He counted 8

times the all GEM

Third test (3D cathode)

3He counted 2.3 times the all

GEM


Focussing to improve resolution

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Bisogna fare il focussing dellultimo esperimento

Counting area ToF [us] FWHM (us)

All GEM ! 12648,66' 229,9'Focussed GEM ! 12714,22' 79,43'

3He! 12673,94' 77,43'

Resolution improves summing spectra of pads that lie on the same Debey-Scherrer cone, i.e. summing the pads that have a constant Lsen


Collimator to improve the S/B ratio

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ToF (us) S/B (BANDGEM collimator) S/B (BANDGEM Cd mask)

Peak 1 6850 1.89 2.34

Peak 3 9200 3.3 2.90

Peak 4 10800 3.23 4.60

BANDGEM Cd maskBANDGEM collimator

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Application of GEM-based detectors to SANS

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BULK PROPERTIES

SANS: Small Angle Neutron Scattering

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MATERIALS length scale ~ [1nm - 1mm]

APPLICATIONS Soft matter

Colloids and polymers Biophysics

Lipids and lipid-protein complexes Hard condensed matter

Superconductors and magnetic materials


The LOKI instrument @ ESS SANS Instrument


LoKI @ ESS

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vacuum

detector

10B4C

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0.5 REQUIREMENTS / < 10% > 30% at 1.8 ToF resolution ~ 100 s

Optimization design of the front

detector

Thanks to A. Jackson, K. Kanaki

1 m

High neutron flux (109 n/cm2/s on sample) Wide solid angle of detector coverage (2)


BAND-GEM for LOKI (Design) The BAND-GEM umbrella is made of 8 single trapezoidal detectors; Each detector is tilted of a specific angle to form an umbrella that covers the

beam; The lamellas are inserted in the horizontal section of each detector.

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tilting angle number of sectors materials (lamellas, frame, glue)

gap between lamellas depth strip dimension

PARAMETERS UNDER STUDY


CAD design of BAND-GEM demonstrator for LoKI

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Cathode

GEM foils

Al lamellas

Anode


Implemented geometry and materials

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Lamellas of Al Lamellas thickness 20 micron strip 3 mm

strips gap 1 mm lamellas pitch 4 mm 1 micron of 10B4C

SOME PARAMETERS

Sample

z

y

LATERAL VIEW

Sample

n beam

dS-D = 10 m

9.6 cm

40 cm

3 cm

FRONT VIEW

x

y


The dgcode coding framework

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dgcode" CODING FRAMEWORK

CROSS-PLATFORM DEPLOYMENT

SIMPLIFYING COLLABORATION BETWEEN MULTIPLE

DEVELOPERSREPOSITORY

hosted by a

Containing codes in C, C++, python, Fortran and BASH organized in folders to provide

units of code or PACKAGES

COLLABORATE ON SOFTWARE TOGETHER WHILE AVOIDING DUPLICATION OF WORK AND (HOPEFULLY) ENSURING MORE TESTED

AND BUG-FREE CODE OVERALL


Simulations

MONOCHROMATIC AND POINTLIKE

BEAM

broadened in

ToF and

Pad dimensionsMaterials

the results will guide the choice of

n beam 2.2

7Li

7Li

7Li

7Li

Tilting angle

dS-D = 10 m

NOT SCALED TO REAL DIMENSIONS !

z

y


Scattering effect on lamellas system (1)

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Simulations parameters

40000 neutrons with same t0 lambda = 2.2 Direction: 2 lamellas hit

zoom

RESULTS A starting deltoid in ToF is splitted in into four peak-shaped arrival timedistributions: 2 layers of B4C for 2 interactions point in the lamellas system

Scattering on Al is minimal

20 m thick Al lamellas 3mm + 1mm empty cathode depth 96mm

Lamella 1

Lamella 2

Layer 2

Layer 1

ToF time spent by n from the first edge in the BAND-GEM to the capture in B4C.


Scattering effect on lamellas system (2)Simulations parameters

40000 neutrons with same t0 lambda = (2 - 12 ) Direction: isotropic on BAND-GEM

20 m thick Al lamellas 3mm + 1mm empty cathode depth 96mm

RESULTS An analytical calculation of the ToF spent by neutron of (2 - 12 ) inside a 96mm depth BANDGEM tells us that the highest ToF is 0.3 ms for neutrons of 12

The simulation is coherent with the analytical calculation

THE TOF BROADENED IS DUE TO THE BAND-GEM GEOMETRY ONLY, NOT TO THE MATERIAL CHOICE


Point Spread Function

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LONG SIDE LAMELLA

STRIPS

+ 7Li

Point: step along the

track

How a point like beam is

broadened in space

LONG SIDE LAMELLA

N.B. lamellas gap is 4mm


FWHM ~ 2 mmFWHM ~ 2 mm

x

y

Point Spread Function

Pads of 4 x 4 mm2


Detector efficiency

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Efficiency: fraction of neutrons converted in and 7Li that deposit energy over threshold wrt the total number of neutrons that hit the detector.

= (attenuation): captured neutrons / tot neutrons that hit the detector

: captured neutrons that deposit energy over threshold / captured neutrons

LoKI REQUIREMENTS

> 30% at =1.8

~ 41% at =1.8


I have done 4 experiments to test the improvements of the GEM detector performances I have implemented a simplified detector geometry inside the ESS SIMULATION FRAMEWORK that allows me to study:

the neutron scattering effect on the lamellas system

the ToF resolution

the detector efficiency

the point spread function

This results are leading in the choice of the parameters in the front detector design (pad dimensions, materials, tilting angle, etc)

A new BAND-GEM prototype will be constructed in the next months e new experimental test will be performed

Conclusions

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The results satisfy the ESS requirements


Future perspectives Implement a complete detector scheme isolating the scattering effect

due to every single material

Implement a Garfield model of the electric field inside the ESS simulation framework to have a complete detector model

Use a real SANS pattern as input in the simulations

Studying the resolution on the relevant parameter for SANS (Q)


Conferences 2015 ECNS 2015, European Conference on Neutron Scattering, Zaragozza

(Spain) , 30 August - 4 September International workshop on IMAGING, Varenna, 7 -10 September SINS annual workshop, ENEA Frascati, July 2015

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Publications G. Albani, et al. Neutron beam imaging with GEM detectors, 2015 JINST

10 C04040 G. Croci, G.Albani, C. Cazzaniga, E. Perelli Cippo, M. Tardocchi and

G.Gorini, Diffraction measurements with a boron-based GEM neutron detector, EPL 107 (2014) 12001

E. Perelli Cippo, G. Croci, A. Muraro, A. Menelle, G. ALbani, A GEM-based thermal neutron detector for high counting rate applications, 2015 JINST 10 P10003


Thank you for your attention!

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Spare slides

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Debye-Scherrer cones

A randomly oriented polycrystalline sample (e.g. a powder) contains a very large number of crystallites

A beam impinging on the sample will find a representative number of crystallites in the right orientation for diffraction

Diffraction occurs only at specific angles, those where Braggs law is satisfied

Giorgia Albani - SoNS seminar

Time of Flight Neutron Diffraction

03/06/14 37

d( !A) = h2mn

t(s)L(m)sin

=1

505.56t(s)

L(m)sin

Time of flight

Scattering angleDistance

d-spacing

Giorgia Albani - Universit Milano Bicocca 38Giorgia Albani - Universit Milano Bicocca

INES: a powder diffractometer

03/06/14 13

Incident n beam

Transmitted n beam

3He tubes Scattered neutrons

sample position

316 K water moderator range: 0,17A-3,24A (E~keV) Large sample holder tank (1m3) 144 diffraction detectors (3He) 9 banks d-spacings: 0.4-12 A Angle range: 11,6- 170,6 High resolution: 0.10% High signal to noise ratio Beam size: 40x40 mm2 Jaws to shape the beam: min 2x2mm2 Laser point to align Sample changer

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