O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Neutron Detectors for Materials Research T.E. Mason Associate Laboratory Director Spallation Neutron Source Acknowledgements: Kent Crawford & Ron Cooper

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 2 Neutron Detectors What does it mean to “detect” a neutron? – Need to produce some sort of measurable quantitative (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 detectors – Gas proportional counters and ionization chambers – Scintillation detectors – Semiconductor detectors

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 3 Nuclear Reactions for Neutron Detectors n + 3 He  3 H + 1 H MeV n + 6 Li  4 He + 3 H MeV n + 10 B  7 Li* + 4 He  7 Li + 4 He MeV  +2.3 MeV(93%)  7 Li + 4 He +2.8 MeV( 7%) n Gd  Gd*   -ray spectrum  conversion electron spectrum n Gd  Gd*   -ray spectrum  conversion electron spectrum n U  fission fragments + ~160 MeV n Pu  fission fragments + ~160 MeV

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 4 Gas Detectors ~25,000 ions and electrons produced per neutron (~4  coulomb)

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 5 Gas Detectors – cont’d Ionization Mode – electrons drift to anode, producing a charge pulse Proportional Mode – if voltage is high enough, electron collisions ionize gas atoms producing even more electrons - gas amplification - gas gains of up to a few thousand are possible

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 6 MAPS Detector Bank

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 7 Scintillation Detectors

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 8 Some Common Scintillators for Neutron Detectors Li glass (Ce) 1.75  %395 nm~7,000 LiI (Eu) 1.83  %470~51,000 ZnS (Ag) - LiF 1.18  % 450 ~160,000 Material Density of 6 Li atoms (cm -3) Scintillation efficiency Photon wavelength (nm) Photons per neutron

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 9 GEM Detector Module

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 10 Anger camera /arb Prototype scintillator-based 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 mm 2

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 11 Semiconductor Detectors

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 12 Semiconductor Detectors cont’d ~1,500,000 holes and electrons produced per neutron (~2.4  coulomb) – This can be detected directly without further amplification – But... standard 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 semiconductor devices?

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 13 Coating with Neutron Absorber Layer must be thin (a few microns) for charged particles to reach detector – detection efficiency is low Most of the deposited energy doesn’t reach detector – poor pulse height discrimination

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 14 Detection Efficiency Full expression: Approximate expression for low efficiency: Where:  = absorption cross-section N = number density of absorber t = thickness N = 2.7  cm -3  atm -1 for a gas For 1-cm thick 3 He at 1 atm and 1.8 Å,  = 0.13

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 15 Pulse Height Discrimination

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 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)

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 17 Position Encoding Discrete - One electrode per position – Discrete detectors – Multi-wire proportional counters (MWPC) – Fiber-optic encoded scintillators (e.g. GEM detectors) Weighted Network (e.g. MAPS LPSDs) – Rise-time encoding – Charge-division encoding – Anger camera Integrating – Photographic film – TV – CCD

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 18 Multi-Wire Proportional Counter Array of discrete detectors Remove walls to get multi-wire counter

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 19 MWPC cont’d Segment the cathode to get x-y position

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 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) – Used on the GLAD and SAND linear PSDs

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 21 Resistive Encoding of a Multi-wire Detector cont’d Position of the event can also be determined from the relative time of arrival of the pulse at the two ends of the resistive network (rise-time encoding) – Used on the POSY1, POSY2, SAD, and SAND PSDs There is a pressurized gas mixture around the electrodes

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 22 Anger camera detector on SCD Photomultiplier outputs are resistively encoded to give x and y coordinates Entire assembly is in a light-tight box

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 23 Micro-Strip Gas Counter Electrodes printed lithgraphically – Small features – high spacial resolution, high field gradients – charge localization and fast recovery

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 24 Crossed-Fiber 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 tube: Phillips XP1704 Coincidence tube: Hamamastu 1924 Resolution: < 5-mm Shaping time: 300 nsec Count rate capability: ~ 1 MHz Time-of-Flight Resolution: 1  sec

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 25 The scintillator screen for this 2-D detector 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 triton and an alpha particle. Collisions with these charged particles cause the ZnS(Ag) to scintillate at a wavelength of approximately 450 nm. The 450 nm 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 1:3. The screen is made by mixing the powders with uncured epoxy and pouring the mix into a mold. The powder then settles to the bottom of the mold before the binder cures. After curing the clear epoxy above the settled powder mix is removed by machining. A mixture containing 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%. Neutron Detector Screen Design

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 26 Neutron Beam Coincidence tube 2-D tube Scintillator Screen Clear Fiber Wavelength-shifting fiber Aluminum wire 16-element WAND Prototype Schematic and Results

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 27 Principle of Crossed-Fiber Position-Sensitive Scintillation Detector Outputs to multi-anode photomultiplier tube Outputs to coincidence single-anode photomultiplier tube 1-mm Square Wavelength-shifting fibers Scintillator screen

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 28 Normalized scattering from 1-cm high germanium crystal E n ~ eV Detector 50-cm from crystal Neutron Scattering from Germanium Crystal Using Crossed-fiber Detector

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 29 All fibers installed and connected to multi- anode photomultiplier mount