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Lecture 4 - Neutron DetectorsLecture 4 Neutron Detectors

MASTER UNIVERSITARIO DI II˚ LIVELLO IN SCIENZE E TECNOLOGIE DEGLI IMPIANTI NUCLEARI

M.Taiuti

Neutron DetectorsWh d i “d ” ?• What does it mean to “detect” a neutron? – need to produce some sort of measurable

i i ( bl ) l i l i lquantitative (countable) electrical signal– can’t directly “detect” slow neutrons

• Need to use nuclear reactions to “convert” neutrons into charged particles

• Then we can use one of the many types of charged particle detectorsg p– gas proportional counters and ionization chambers– scintillation detectors– semiconductor detectors 2

Nuclear Reactions for Neutron Detectors

• n + 3He → 3H + 1H + 0 764 MeV• n + 3He → 3H + 1H + 0.764 MeV• n + 6Li → 4He + 3H + 4.79 MeV• n + 10B → 7Li* + 4He →7Li + 4He + 0.48 MeV γ + γ

2.3 MeV (93%)→ 7Li + 4He +2.8 MeV (7%)

• n + Cd → Cd* → γ-ray spectrum• n + Cd → Cd → γ-ray spectrum• n + 155Gd → Gd* → γ-ray spectrum• n + 157Gd → Gd* → γ-ray spectrum γ• n + 235U → fission fragments + ~160 MeV • n + 239Pu → fission fragments + ~160 MeV

197 ( ) 115 ( ) 181 ( ) 238 (• 197Au(4.906 eV), 115In( 1.46 eV), 181Ta(4.28 eV), 238U(6.67, 10.25 eV); energy-selective detectors, narrow resonances, prompt capture gamma rays

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Converter Materials3H i i it bl i th t t ll t• 3He is inevitably in the gaseous state, usually at several atmospheres pressure6Li is usually in chemically combined form e g LiF• 6Li is usually in chemically combined form, e.g., LiF, Li2O, Li6Gd(BO3)3, or in chemical compounds combined heterogeneously or homogeneously withcombined heterogeneously or homogeneously with intrinsic scintillators

• B is usually in chemically combined form, e.g., BF3y y , g , 3 (gas) or H3 BO3 in solution, sometimes as a thin coating in elemental form

• Gd as a converter is usually as the oxide or silicate• U is usually thinly coated in oxide form, Ta in thin foil

Lecture 4 - Neutron Detectors 4

http://www.nndc.bnl.gov/exfor/endf00.jsphttp://www.nndc.bnl.gov/exfor/endf00.jsp

Cross Sections

n + 3He → 3H + 1Hn + 6Li

104104

106106

106106

1041041010

102102102102

n + 10Bn + 112Cd

104104

106106

104104 102102

102102

5

Cross Sections

n + 157Gdn + 155Gd

106106

106106

104104104104

102102 1021021010 102102

n + 235U n + 239Pu104104 104104

106106106106

102102 102102

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An ExerciseWh i h l h f l i i i• What is the wavelength of a non-relativistic neutron with kinetic energy T?


Semiconductor Detectors

n Li He H MeV+ → + +6 4 3 4 79.

8Lecture 4 - Neutron Detectors

Semiconductor Detectors cont’d1 500 000 h l d l d d• ~1,500,000 holes and electrons produced per

neutron (~2.4×10-13 Coulomb)– this can be detected directly without further

amplificationb d d d d d– but . . . standard device semiconductors do not contain enough neutron-absorbing nuclei to give reasonable neutron detection efficiencyreasonable neutron detection efficiency

• put neutron absorber on surface of semiconductor?• develop boron phosphide semiconductor devices?p p p


Coating with Neutron Absorber

• Layer must be thin (a few microns) for charged i l h dparticles to reach detector

– detection efficiency is low

Most of the deposited energy doesn’t reach detector• Most of the deposited energy doesn’t reach detector – poor pulse height discrimination 10

Gas Detectors

n He H H MeV+ → + +3 3 1 0 76.

~25,000 ions and electrons produced per neutron (~4×10-15

Coulomb)


Gas DetectorsI i ti M d

- ++-Neutron

Heavy particle (M1) range

Neutron capture event

• Ionization Mode– electrons drift to anode,

producing a charge pulse

++--

++

+-

-+-

-

Neutron*

+++

+- -

+

Light particle (M2) range

Ionization producing a charge pulse with no gas multiplication

– typically employed in l ffi i b it d t t

--- +++--tracks

low-efficiency beam-monitor detectors

• Proportional Modeif voltage is high enough electron collisions ionize– if voltage is high enough, electron collisions ionize gas atoms producing even more electrons

• gas amplification increases the collected charge• gas amplification increases the collected charge proportional to the initial charge produced.

• gas gains of up to a few thousand are possible, above hich p opo tionalit is lostwhich proportionality is lost.

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Fission ChamberFission chambers use neutron induced fission to detect neutrons The chamber• Fission chambers use neutron-induced fission to detect neutrons. The chamber is usually similar in construction to that of an ionization chamber, except that the coating material is highly enriched 235U. The neutrons interact with the 235U, causing fission. One of the two fission fragments enters the chamber, while the g g ,other fission fragment embeds itself in the chamber wall.

• One advantage of using 235U coating rather than boron is that the fission fragment has a much higher energy level than the α particle from a boron

i N i d d fi i f d i i i ireaction. Neutron-induced fission fragments produce many more ionizations in the chamber per interaction than do the neutron- induced alpha particles. This allows the fission chambers to operate in higher gamma fields than an uncompensated ion chamber with boron lining.uncompensated ion chamber with boron lining.

200 mm

Cathode Fissile material Ionizing gas(Ar + 5%N2)

El t i i l t

Cathode Fissile material Ionizing gas(Ar + 5%N2)

Electric insulator Anode Housing Double coaxial MI

φ14m

m


Electric insulatorElectric insulator Anode Housing Double coaxial MICable φ6 mm

Gas Detectors P ti l t (PC ) i i t f diff t f• Proportional counters (PCs) come in a variety of different forms.

• Simple detector (shown previously)

• Linear position-sensitive detector (LPSD):Linear position sensitive detector (LPSD):– the anode is resistive, read out from both ends—the charge

distributes between the ends according to the position of the neutron capture event in the tube.neutron capture event in the tube.

– usually cylindrical.

• 2-d position-sensitive detector (MWPC)– many parallel resistive wires extend across a large thick area of fill

gas. Each wire operates either as in LPSD or without position information as in a simple PC

OR– two mutually perpendicular arrays of anode wires. Each is read

separately as an LPSD to give two coordinates for the neutron p y gcapture event

– MWPCs usually have a planar configuration 14

Reuter-Stokes LPSD

15

Pulse Height Discrimination


Pulse Height Discrimination gcont’d

d l l• Can set discriminator levels to reject undesired events (fast neutrons, ( ,gammas, electronic noise)

• Pulse height discrimination can make a• Pulse-height discrimination can make a large improvement in background

• Discrimination capabilities are an important criterion in the choice of important criterion in the choice of detectors (3He gas detectors are very good)good)


2-d DetectorMAPS Detector Bank


Multi-Wire Proportional Counter

• Array of discrete detectors

• Remove walls to get multi-wire counter• Remove walls to get multi-wire counter19Lecture 4 - Neutron Detectors

MWPC cont’d

• Segment the cathode to get x-y i iposition


Resistive Encoding of a gMulti-wire Detector

• Instead of reading every cathode strip individually, th t i b i ti l l d ( h &the strips can be resistively coupled (cheaper & slower)

• Position of the event can be determined from the• Position of the event can be determined from the fraction of the charge reaching each end of the resistive network (charge-division encoding)resistive network (charge division encoding)

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Resistive Encoding of a gMulti-wire Detector cont’d

P i i f h• Position of the event can also be determined from h l i i f i lthe relative time of arrival

of the pulse at the two ends f th i ti t kof the resistive network

(rise-time encoding) • There is a pressurized gas

mixture around the electrodes


Micro-Strip Gas Counter

Drift Cathode

Insulator

Cathode StripsElectrostatic Field Lines

High Field (Avalanche) Region

Anode StripsCollector Anode

• Electrodes printed lithographically– small features – high spacial resolution, high fieldsmall features high spacial resolution, high field

gradients – charge localization and fast recovery

Sizes of Proportional CountersPC d LPSD i i• PCs and LPSDs come in many sizes.– Diameters from ~ 5. mm to 50 mm.

Fill hi h t f ll di t t– Fill gas pressures are highest for small diameters, up to 40 atm, and lowest for large diameters 2.~ 3. atm

– Lengths vary from cm to meters; the longer detectorsLengths vary from cm to meters; the longer detectors, up to about 3. m long, are typically those of larger diameter

• MWPCs are usually flat and square, but sometimes rectangular, even curved, or banana-shaped– Typical dimension 0.5 ~ 1.0 m


Detection EfficiencyD t t l i t ll th i id t t• Detectors rarely register all the incident neutrons. The ratio of the number registered to the number incident is the efficiencyincident is the efficiency

• Full expression:• Approximate expression for low efficiency:• Approximate expression for low efficiency:• Where:

σ = absorption cross-sectionσ absorption cross sectionN = number density of absorbert = thicknessN 2 7 1019 3 t 1 fN = 2.7・1019 cm-3 ・ atm-1 for a gas

• For 1 cm thick 3He at 1 atm and 1.8 Å neutrons:

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Spatial ResolutionS i l l i (h ll h d ll• Spatial resolution (how well the detector tells the location of an event) is always limited by h h d i l d b hthe charged-particle range and by the range

of neutrons in the fill gas, which depend on th d iti f th fillthe pressure and composition of the fill gas

• And by the geometry:– simple PCs: δ z ~ diameter; 6 mm - 50 mm– LPSDs: δ z ~ diameter, δ y ~ diameter ; 6 mm -

50 mm– MWPC: δ z and δ y ~ wire spacing; 1 mm - 10 mm


Scintillation DetectorsLi ht t t f li id i i till t• Light output from liquid organic scintillators shows different pulse shape for γ and fast neutrons:neutrons:– γ interacts by photoelectric, Compton and pair

productionproduction– neutron interacts by scattering on proton, transferring

part of the kinetic energy


Scintillation Detectors

n Li He H MeV+ → + +6 4 3 4 79.

28

Some Common Scintillators for Neutron Detectors

Z S(A ) i th b i ht t i till t k i t i i• ZnS(Ag) is the brightest scintillator known, an intrinsic scintillator that is mixed heterogeneously with converter material, usually Li6F in the “Stedman” recipe, to form scintillating composites. These are only semitransparent. But it is somewhat slow, decaying with ~10 μsec halftime

• GS-20 (glass,Ce3+) is mixed with a high concentration of Li2O inGS 20 (glass,Ce ) is mixed with a high concentration of Li2O in the melt to form a material transparent to light

• Li6Gd(BO3)3 (Ce3+) (including 158Gd and 160Gd, 6Li ,and 11B), and 6LiF(E ) i t i i i till t th t t i hi h ti f6LiF(Eu) are intrinsic scintillators that contain high proportions of converter material and are typically transparent

• An efficient gamma ray detector with little sensitivity to g y yneutrons, used in conjunction with neutron capture gamma-ray converters, is YAP (yttrium aluminum perovskite, YAl2O3(Ce3+))


Some Common Scintillators for Neutron Detectors

MaterialDensity of6Li atoms(cm-3)

Scintillationefficiency

Photonwavelength

(nm)

Photons per neutron

Li glass (Ce) 1.75×1022 0.45 % 395 ~7,000

(cm 3) (nm)

LiI (Eu) 1.83×1022 2.8 % 470 ~51,000

Z S (A ) LiF 1 18 1022 9 2 % 450 160 000ZnS (Ag) - LiF 1.18×1022 9.2 % 450 ~160,000

Li6Gd(BO3)3 (Ce), 3.3x1022 ~40,000~ 400

YAP ~18,000per MeV γ

350NA


Anger Camera Detector

• Photomultiplier outputs are resistively encoded to give x and y coordinates g y

• Entire assembly is in a light-tight box 31

Anger Camera DetectorP t t i till t b d• Prototype scintillator-based area-position-sensitive neutron detectorneutron detector

• Designed to allow easy expansion into a 7x7expansion into a 7x7 photomultiplier array with a 15x15 cm2 active scintillator area.

• Resolution is expected to be 1 5 1 5 2~1.5x1.5 mm2


Crossed-Fiber Position-SensitiveCrossed Fiber Position Sensitive Scintillation Detector

Outputs to multi-anode photomultiplier tube

1-mm Square Wavelength-shifting fibers

Outputs to coincidence single-anode photomultiplier tube

Scintillator screen


Th i till t f thi 2 d d t t i t f i t• The scintillator screen for this 2-d detector consists of a mixture of 6LiF and silver-activated ZnS powder in an epoxy binder

• Neutrons incident on the screen react with the 6Li to produce a t and an α particle. Collisions with these charged particles cause the ZnS(Ag) to scintillate at a wavelength of approximately 450the ZnS(Ag) to scintillate at a wavelength of approximately 450 nm. The photons are absorbed in the wavelength-shifting fibers where they converted to 520 nm photons emitted in modes that propagate out the ends of the fiberspropagate out the ends of the fibers

• The optimum mass ratio of 6LiF:ZnS(Ag) was determined to be 1 3 A i t t i i 40 / 2 f 6LiF d 120 / 2 f1:3. A mixture containing 40 mg/cm2 of 6LiF and 120 mg/cm2 of ZnS(Ag) is used in this screen design. This mixture has a measured neutron conversion efficiency of over 90%



D i P (ORNL I&C)• Design Parameters (ORNL I&C) – size: 25 cm x 25 cm– thickness: 2 mm– number of fibers: 48 for each axis

lti d h t lti li t b Philli XP1704– multi-anode photomultiplier tube: Phillips XP1704– coincidence tube: Hamamatsu 1924

resolution: < 5 mm– resolution: < 5 mm– shaping time: 300 nsec– count rate capability: ~ 1 MHzcount rate capability: ~ 1 MHz– time-of-Flight Resolution: 1 μsec

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