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SNS Experimental FacilitiesOak Ridge X0000910/arb Neutron Detectors for Materials Research T.E. Mason Experimental Facilities Division Spallation Neutron.

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Presentation on theme: "SNS Experimental FacilitiesOak Ridge X0000910/arb Neutron Detectors for Materials Research T.E. Mason Experimental Facilities Division Spallation Neutron."— Presentation transcript:

1 SNS Experimental FacilitiesOak Ridge X /arb Neutron Detectors for Materials Research T.E. Mason Experimental Facilities Division Spallation Neutron Source Acknowledgements: Kent Crawford & Ron Cooper

2 SNS Experimental FacilitiesOak Ridge X /arb 2 Neutron Detectors What does it mean to detect a neutron? – Need to produce some sort of measurable quantitative (countable) electrical signal – Cant 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

3 SNS Experimental FacilitiesOak Ridge X /arb 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

4 SNS Experimental FacilitiesOak Ridge X /arb 4 Gas Detectors ~25,000 ions and electrons produced per neutron (~ coulomb)

5 SNS Experimental FacilitiesOak Ridge X /arb 5 Gas Detectors – contd 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

6 SNS Experimental FacilitiesOak Ridge X /arb 6 MAPS Detector Bank

7 SNS Experimental FacilitiesOak Ridge X /arb 7 Scintillation Detectors

8 SNS Experimental FacilitiesOak Ridge X /arb 8 Some Common Scintillators for Neutron Detectors Li glass (Ce) %395 nm~7,000 LiI (Eu) %470~51,000 ZnS (Ag) - LiF % 450 ~160,000 Material Density of 6 Li atoms (cm -3) Scintillation efficiency Photon wavelength (nm) Photons per neutron

9 SNS Experimental FacilitiesOak Ridge X /arb 9 GEM Detector Module

10 SNS Experimental FacilitiesOak Ridge X /arb 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

11 SNS Experimental FacilitiesOak Ridge X /arb 11 Semiconductor Detectors

12 SNS Experimental FacilitiesOak Ridge X /arb 12 Semiconductor Detectors contd ~1,500,000 holes and electrons produced per neutron (~ 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?

13 SNS Experimental FacilitiesOak Ridge X /arb 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 doesnt reach detector – poor pulse height discrimination

14 SNS Experimental FacilitiesOak Ridge X /arb 14 Detection Efficiency Full expression: Approximate expression for low efficiency: Where: = absorption cross-section N = number density of absorber t = thickness N = cm -3 atm -1 for a gas For 1-cm thick 3 He at 1 atm and 1.8 Å, = 0.13

15 SNS Experimental FacilitiesOak Ridge X /arb 15 Pulse Height Discrimination

16 SNS Experimental FacilitiesOak Ridge X /arb 16 Pulse Height Discrimination contd 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)

17 SNS Experimental FacilitiesOak Ridge X /arb 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

18 SNS Experimental FacilitiesOak Ridge X /arb 18 Multi-Wire Proportional Counter Array of discrete detectors Remove walls to get multi-wire counter

19 SNS Experimental FacilitiesOak Ridge X /arb 19 MWPC contd Segment the cathode to get x-y position

20 SNS Experimental FacilitiesOak Ridge X /arb 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

21 SNS Experimental FacilitiesOak Ridge X /arb 21 Resistive Encoding of a Multi-wire Detector contd 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

22 SNS Experimental FacilitiesOak Ridge X /arb 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

23 SNS Experimental FacilitiesOak Ridge X /arb 23 Micro-Strip Gas Counter Electrodes printed lithgraphically – Small features – high spacial resolution, high field gradients – charge localization and fast recovery

24 SNS Experimental FacilitiesOak Ridge X /arb 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

25 SNS Experimental FacilitiesOak Ridge X /arb 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

26 SNS Experimental FacilitiesOak Ridge X /arb 26 Neutron Beam Coincidence tube 2-D tube Scintillator Screen Clear Fiber Wavelength-shifting fiber Aluminum wire 16-element WAND Prototype Schematic and Results

27 SNS Experimental FacilitiesOak Ridge X /arb 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

28 SNS Experimental FacilitiesOak Ridge X /arb 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

29 SNS Experimental FacilitiesOak Ridge X /arb 29 All fibers installed and connected to multi- anode photomultiplier mount


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