Neutron Detection 22.104 Spring 2002
Neutrons vs. X-rays
Ideal Large Detector Pulse Height
Neutron Interactions
Total Cross section for Moderators
Neutron Slowing Down
Neutron Histories in Moderated Detector
Inelastic Neutron Scattering
Elastic Scattering Kinematics
Elastic Scattering Kinematics
Recoil Proton Energy for Monenergetic Neutrons
Neutron Moderator Energy Distributions
Pulse Height vs. Energy in Plastic
Recoil Spectrum Distortion in Plastic
Fission Cross section for Slow Neutrons
Fission Cross section for Fast Neutrons
Fission Fragments
Cross section for Neutron Detector Reactions
Some Reactions for Neutron Detectors n+ 3 He p+ 3 H + 765 kev n+ 10 5 B á + 7 3 Li + 2.310 MeV (94%) á + 7 3 Li + 2.792 MeV (6%) n+ 6 Li 3 H + á + ~ 4.8 MeV
He-3 and Li-6 Cross sections
He-3 Detector Spectrum
BF-3 Pulse Heights
Wall Effects
Degradation of BF3 Counters
Neutron Scintillator Choices Thermal ( < 0.5 ev) load scintillator with materials with high capture crosssection such as Li. B, Gd Epithermal (0.5 ev - 50 kev) Moderate neutrons and then use thermal techniques Fast neutrons (> 50 kev) detect through (n,p) or (n,d) elastic scatters hydrogen rich materials such as plastic scintillators
Neutron Scintillators
Li Scintillators - Glass and Phosphors Neutron Energy Glass Type Glass Thickness 0.01-20 ev All types 0.1-10 mm Thermal GS20 1.3 mm 10 ev - 100 KeV GS20 3.2 mm 100 ev - 1 MeV GS20 25.4 mm 1-600 KeV KG2, GS20 9.5 mm 1-6 Mev GS20 25 mm 6 3 n+ Li H + α + ~ 4.8 MeV
Li Glass Pulse Height Response
Applications of Neutron Detection Reactors
PWR Out of Core Instruments
PWR Ranges
Instrument Requirements Start-up gamma Flux dominates require discrimination against gammas pulse mode possible Intermediate gamma still important switch to current mode reject gammas» CIC» MSV Full Power gammas not important simplicity for safety ion chambers
Compensated Ion Chamber
Mean Square Value Counter
BWR Ranges
BWR In-Core Fission Counter
Memory Effects
Operating Voltages
Self powered Neutron Detector
Applications of Neutron Detection Radiography
707 Body Panel
Fast Neutron Radiograph
C, O, and N Cross-sections
Nitrogen Cross-section
Early Experiment at NAC Cross section: barns 18 16 14 12 10 8 6 4 2 Total Neutron Cross Section H1 C12 N14 O16 0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 En: MeV Detecting explosives by taking difference of two radiographic images. The contrast comes from nitrogen and oxygen.
High Resolution Neutron Images
Resonant Images of Carbon
Neutron Resonance Radiography 9 Figure 1. Total Neutron Cross Section 8 7 H1 C12 N14 O16 Cross Section: barns 6 5 4 3 2 N off N on C off O on C on O off 1 0 0.5 1 1.5 2 2.5 En: Mev
Resonance Radiography Drug simulant 50 100 150 200 Explosive simulant En=2.37MeV H:1.5g C:16.3g N:0.9g O:5.2g H:1.0g C:7.1g N:7.1g O:12.3g C:10.6g H:1.8g C:10.6g N:24.9g 50 100 150 200 O-At-Valley/O-On-Res. Graphite 50 100 150 Melamine 200 N-Off-Res./N-On-Res. 50 100 150 200 C-Off-Res./C-On-Res. 50 100 50 100 150 150 200 200 50 100 150 200 50 100 150 200
Composite Picture
D-D Kinematics 10 8 Outgoing Neturon Energy (MeV) 6 4 2 0 0 1 2 3 4 5 6 7 8 9 10 Incoming Deuteron Energy (MeV)
Using a D-D Neutron Source Ed = 2.3MeV Ed=0.8MeV (thick target) Different energies are obtained by rotating object-detector assembly around the source 6.5 6 5.5 5 0 30 60 90 120 D-D Neutron Energy at Different Angles Deuteron Target 0 o En: MeV 4.5 4 40 o 25 o 3.5 3 2.5 2 0 0.5 1 1.5 2 2.5 3 Ed: MeV 115 o 100 o 90 o 80 o 70 o 50 o 60 o Object Detector
A terrorist overnight bag Simulation of NRR Bag size: 40 X 30 X 10 cm. Contents of interest: a bag of sugar (105g), a stash of cocaine- HCl (105g), a paperback book, a block of plastic explosive (270 g). Heavily loaded with various of items and has an average density of around 0.5g/cm 3. Source-Object: ~150cm; Object-Detector: ~50cm.
Neutron and X-Ray Images Neutron Image(0 degree) X-Ray Image(140kev) 50 50 100 100 150 150 200 50 100 150 200 200 50 100 150 200 Simulations were run on Blue Pacific supercomputer at Lawrence Livermore National Laboratory, with 5x10 8 source particles for each image.
Elemental Mapping
All Element Mapping Other Elements Mapping 0.3 All Elements Mapping 0.6 0.25 0.5 50 0.2 50 0.4 100 0.15 0.1 100 0.3 0.2 150 0.05 150 0.1 200 50 100 150 200 0-0.05 200 50 100 150 200 0 Elements other than H, C, N, O are reported as one component All-elements mapping is the total projected content. (atoms/cm 2)
Detecting Mines