Radiation Monitoring Systems & Waste Characterization Lecture 4 - Neutron Detectors M.Taiuti MASTER UNIVERSITARIO DI II LIVELLO IN SCIENZE E TECNOLOGIE DEGLI IMPIANTI NUCLEARI
Neutron Detectors What does it mean to detect a neutron? need to produce some sort of measurable quantitative i (countable) electrical l 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 detectors gas proportional counters and ionization chambers scintillation detectors semiconductor detectors 2
Nuclear Reactions for Neutron Detectors n+ 3 He 3 H+ 1 H + 0.764 MeV n + 6 Li 4 He + 3 H + 4.79 MeV n + 10 B 7 Li* + 4 He 7 Li + 4 He + 0.48 MeV γ + 2.3 MeV (93%) 7 Li + 4 He +2.8 MeV (7%) n+cd Cd* γ-ray spectrum n + 155 Gd Gd* γ-ray spectrum n + 157 Gd Gd* γ-ray spectrum n + 235 U fission fragments + ~160 MeV n + 239 Pu fission fragments + ~160 MeV 197 Au(4.906 ev), 115 In( 1.46 ev), 181 Ta(4.28 ev), 238 U(6.67, 10.25 ev); energy-selective detectors, narrow resonances, prompt capture gamma rays 3
Converter Materials 3 He is inevitably it in the gaseous state, t usually at several atmospheres pressure 6 Li is usually in chemically combined form, e.g., LiF, Li 2 O, Li 6 Gd(BO 3 ) 3, or in chemical compounds combined heterogeneously or homogeneously with intrinsic scintillators B is usually in chemically combined form, e.g., BF 3 (gas) or H 3 BO 3 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.jsp Cross Sections n + 3 He 3 H + 1 H 106 6 10 6 n + 6 Li 10 4 10 4 10 2 10 2 n + 10 B 10 6 n + 112 Cd 10 4 10 4 10 2 10 2 5
Cross Sections n + 155 Gd n + 157 Gd 10 6 10 6 10 4 10 4 10 2 10 2 n + 235 U 10 6 n + 239 Pu 10 6 10 4 10 4 10 2 10 2 6
An Exercise What is the wavelength of a non-relativistic i i neutron with kinetic energy T? Lecture 4 - Neutron Detectors 7
Semiconductor Detectors n + 6 Li 4 He+ 3 H+ 479. MeV Lecture 4 - Neutron Detectors 8
Semiconductor Detectors cont d ~1,500,000 000 holes and electrons produced d per neutron (~2.4 10-13 Coulomb) this can be detected directly without further amplification but... standard d device semiconductors do not contain enough neutron-absorbing nuclei to give reasonable neutron detection efficiency put neutron absorber on surface of semiconductor? develop boron phosphide p semiconductor devices? Lecture 4 - Neutron Detectors 9
Coating with Neutron Absorber Layer must be thin (a few microns) for charged particles il to reach hdetector detection efficiency is low Most of the deposited energy doesn t reach detector poor pulse height discrimination 10
Gas Detectors n + 3 He 3 H+ 1 H+ 076. MeV ~25,000 ions and electrons produced per neutron (~4 10-15 Coulomb) Lecture 4 - Neutron Detectors 11
Gas Detectors Ionization Mode Neutron electrons drift to anode, producing a charge pulse with no gas multiplication typically employed in low-efficiency i beam-monitor detectors t Proportional Mode Heavy particle (M 1 ) range + - - + - ++- -+- * + + + - - - + Ionization - + - - + tracks - - + - + Neutron capture event Light particle (M 2 ) range if voltage is high enough, electron collisions ionize gas atoms producing even more electrons gas amplification increases the collected charge proportional to the initial charge produced. gas gains of up to a few thousand are possible, above which proportionality is lost. 12
Fission 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 235 U. The neutrons interact with the 235 U, causing fission. One of the two fission fragments enters the chamber, while the other fission fragment embeds itself in the chamber wall. One advantage of using 235 U coating rather than boron is that the fission fragment has a much higher energy level than the α particle from a boron reaction. Neutron-induced d fission i fragments produce many more ionizations i i 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. φ14mm 200 mm Cathode Fissile material Ionizing gas(ar + 5%N2) Electric insulator Anode Housing Double coaxial MI Electric insulator Cable φ6 mm Lecture 4 - Neutron Detectors 13
Gas Detectors Proportional counters (PCs) come in a variety of different forms. Simple detector (shown previously) Linear position-sensitivesensitive 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. 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 capture event MWPCs usually have a planar configuration 14
Reuter-Stokes LPSD 15
Pulse Height Discrimination Lecture 4 - Neutron Detectors 16
Pulse Height Discrimination cont d Can set discriminator levels to reject undesired events (fast neutrons, gammas, electronic noise) Pulse-height discrimination can make a large improvement in background Discrimination capabilities are an important criterion in the choice of detectors ( 3 He gas detectors are very good) Lecture 4 - Neutron Detectors 17
2-d Detector MAPS Detector Bank Lecture 4 - Neutron Detectors 18
Multi-Wire Proportional Counter Array of discrete detectors Remove walls to get multi-wire counter Lecture 4 - Neutron Detectors 19
MWPC cont d Segment the cathode to get x-y position ii Lecture 4 - Neutron Detectors 20
Resistive Encoding of a Multi-wire Detector Instead of reading every cathode strip individually, the strips can be resistively coupled (cheaper & slower) Position of the event can be determined from the fraction of the charge reaching each end of the resistive network (charge-division encoding) 21
Resistive Encoding of a Multi-wire Detector cont d Position i of the event can also be determined from the relative time of arrival of the pulse at the two ends of fthe resistive network (rise-time encoding) There is a pressurized gas mixture around the electrodes Lecture 4 - Neutron Detectors 22
Micro-Strip Gas Counter Drift Cathode Electrostatic Field Lines High Field (Avalanche) Region Cathode Strips Insulator Collector Anode Anode Strips Electrodes printed lithographically small features high spacial resolution, high field gradients charge localization and fast recovery
Sizes of Proportional Counters PCs and LPSDs come in many sizes. Diameters from ~ 5. mm to 50 mm. Fill gas pressures are highest h for small diameters, up to 40 atm, and lowest for large diameters 2.~ 3. atm Lengths 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 bananashaped Typical dimension 0.5 ~ 1.0 m Lecture 4 - Neutron Detectors 24
Detection Efficiency Detectors t rarely register all the incident id neutrons. The ratio of the number registered to the number incident is the efficiency Full expression: Approximate expression for low efficiency: Where: σ = absorption cross-sectionsection N = number density of absorber t = thickness N = 27 2.7 10 19 cm -3 atm -1 for a gas For 1 cm thick 3 He at 1 atm and 1.8 Å neutrons: 25
Spatial Resolution Spatial resolution (how well the detector tells the location of an event) is always limited by the charged-particle il range and dby the range of neutrons in the fill gas, which depend on the 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 Lecture 4 - Neutron Detectors 26
Scintillation Detectors Light output t from liquid id organic scintillators t shows different pulse shape for γ and fast neutrons: γ interacts by photoelectric, Compton and pair production neutron interacts by scattering on proton, transferring part of the kinetic energy Lecture 4 - Neutron Detectors 27
Scintillation Detectors n + 6 Li 4 He+ 3 H+ 479. MeV 28
Some Common Scintillators for Neutron Detectors ZnS(Ag) is the brightest t scintillator t known, an intrinsic i i scintillator that is mixed heterogeneously with converter material, usually Li 6 F 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,ce 3+ ) is mixed with a high concentration of Li 2 Oin the melt to form a material transparent to light Li 6 Gd(BO 3 ) 3 (Ce 3+ ) (including 158 Gd and 160 Gd, 6 Li,and 11 B), and 6 LiF(Eu) are intrinsic i i scintillators t that t contain high h proportions of converter material and are typically transparent An efficient gamma ray detector with little sensitivity to neutrons, used in conjunction with neutron capture gamma-ray converters, is YAP (yttrium aluminum perovskite, YAl 2 O 3 (Ce 3+ )) Lecture 4 - Neutron Detectors 29
Some Common Scintillators for Neutron Detectors Material Density of 6 Li atoms (cm -3) Scintillation efficiency Photon wavelength (nm) Photons per neutron Li glass (Ce) 1.75 10 22 0.45 % 395 ~7,000 LiI (Eu) 1.83 10 22 2.8 % 470 ~51,000 ZnS (Ag) - LiF 118 1.18 1010 22 92% 9.2 450 ~160,000 000 Li 6 Gd(BO 3 ) 3 (Ce), 3.3x10 22 ~40,000 ~ 400 YAP NA 350 ~18,000 per MeV γ Lecture 4 - Neutron Detectors 30
Anger Camera Detector Photomultiplier outputs are resistively encoded to give x and y coordinates Entire assembly is in a light-tight box 31
Anger Camera Detector Prototype t scintillator-based t area-position-sensitive neutron detector Designed to allow easy expansion into a 7x7 photomultiplier array with a 15x15 cm 2 active scintillator area. Resolution is expected to be ~1.5x1.5 15 15mm 2 Lecture 4 - Neutron Detectors 32
Crossed-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
Crossed-Fiber Position-Sensitive Scintillation Detector The scintillator t screen for this 2-d ddetector t consists of a mixture of 6 LiF and silver-activated ZnS powder in an epoxy binder Neutrons incident on the screen react with the 6 Li to produce a t and an α particle. Collisions with these charged particles cause the 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 fibers The optimum mass ratio of 6 LiF:ZnS(Ag) was determined to be 13 1:3. A mixture containing i 40 mg/cm 2 of 6 LiF and 120 mg/cm 2 of ZnS(Ag) is used in this screen design. This mixture has a measured neutron conversion efficiency of over 90% Lecture 4 - Neutron Detectors 34
Crossed-Fiber Position-Sensitive Scintillation Detector Design Parameters (ORNL I&C) size: 25 cm x 25 cm thickness: 2 mm number of fibers: 48 for each axis multi-anode photomultiplier li tube: Phillips XP1704 coincidence tube: Hamamatsu 1924 resolution: < 5 mm shaping time: 300 nsec count rate capability: ~ 1 MHz time-of-flight Resolution: 1 μsec 35
Crossed-Fiber Position-Sensitive Scintillation Detector Lecture 4 - Neutron Detectors 36