Nuclear data and modelling for monitoring compliance with the Comprehensive Test Ban Treaty

Transcription

1 Nucear data and modeing for monitoring compiance with the Comprehensive Test Ban Treaty R Britton, A Davies UK Radionucide Laboratory GBL15 rich.britton@awe.co.uk, ashey.davies@awe.co.uk

2 Overview Introduction CTBTO GBL15 Exampe Measurements Key Radionucide signatures Compton Suppression Cosmic Suppression Monte-Caro Modeing Detector Characterisations Design Studies Summary

3 The CTBT bans a nucear exposions on Earth whether for miitary or peacefu purposes Verification Technoogies

4 The Internationa Monitoring System RN68 Tristan da Cunha RN67 St Heena RN69 Antarctica RN66 BIOT

5 GBL15 AWE hosts the UK Radionucide aboratory (GBL15) in support of the CTBT Many years experience of carrying out radionucide measurements and diagnosis supporting UK s nucear test programme (AGT/UGTs) Sampes were sent from distant nucear test sites for anaysis of fission and activation products, and residua device materias 2004 certified by CTBTO as GBL15 and part of Internationa Monitoring System Utra ow-background detectors for routine anaysis 2 x BE5030, 2 x BE6530, 1 x 93% eff. extended p-type Research and non-ctbt detectors 3 x Compton suppression, 1 x cosmic veto, 1 x Gamma-Gamma coincidence system, 10 x HPGe

6 Radioogica and Nucear Forensics To further deveop capabiities from pre and post-detonation scenarios What is it? Where is it from? Who is invoved? Where has it been?

7 Laboratory instrumentation Apha spectrometry Apha Anayst (x 24 PIPS) Portabe units (x 3 dua) Apha/Beta imatics (x 3) isoo (x 3) Liquid scintiation Quantuus 1220 Radon mobie aboratory SAUNA Radioxenon Laboratory

8 Key Radionucide Signatures Three types of reease in a nucear exposion fue products, fission products and activation products Detection probabiities must consider: The amount of RA materia reeased The haf-ife of the radioisotopes The type & branching ratios of the decay radiation Key nucides incude Pu, Cs, Ba, La, Np, I, Zr, Mo, Tc, Ru, Ce, Am

9 Research systems

10 Cosmic veto Athough ead shieding reduces the gamma-background, it increases cosmic-ray interactions and the cosmicbackground. Reduction of this component is achievabe by: 1.Instaation of the gamma-spectrometer in an underground aboratory 2.Increasing the overburden above the detector 3.Using a cosmic veto device such as pastic scintiation pates surrounding the ead shieding in anti-coincidence with the primary detector Counts A gamma-spectrum shows peaks at 595.9, and kev attributabe to prompt neutron capture reactions. The spectra was coected over 63 days Energy (MeV) A cosmic radiation spectrum measured using a BC408 pastic detector. Davies AV, Burnett JL (2012) JRNC, Vo 292-3, pp 1007

11 Cosmic veto Version 3.0 Hardware gating using two Lynx MCAs Canberra extended p-type HPGe detector within ow-background graded shied Surrounded by five 55 cm x 55 cm Bicron BC408 pastic scintiation pates. Combined contro using Canberra Lynx MCAs with pre-ampifiers. Hardware gating for rea-time processing.. Gate Inside an extended range p-type detector HPGe ICR Scintiation pates

12 Background reduction 7-day background spectra reduced by a mean of 75.2%. Reduction of Ge neutron capture peaks by %. Davies AV, Burnett JL (2013) JRNC, Vo 298-2, pp 987

13 MDA improvement During 5-day count, MDA for 85 CTBT radionucides improved by % with mean of 45.6%. Ba-140 MDA of 19.2 mbq (2 days) and 11.3 mbq (7 days). This compares to 35.9 mbq and 21.7 mbq. Ba-140 MDA of 24 mbq at 1.5 days. MDAs for Zr-95, Mo-99, Ce-141 and Nd-147 improved by 52.7%, 37.3%, 38.5% and 52.1% after 7 days. MDAs for I-131, Cs-134 and Cs-137 improved by 42.0%, 46.6% and 49.5%. MDA improvements for the 85 CTBT radionucides after 5 days. Some key indicators of nucear weapons tests (back diamonds) and the Fukushima incident (grey squares) are abeed. Davies AV, Burnett JL (2013) JRNC, Vo 298-2, pp 987

14 Upgrade of RN67 St Heena

15 What is Compton suppression? Typicay consists of a high-resoution Ge detector couped with an anti-compton NaI(T) shied. The shied aows the veto of Compton scattered events, where the fu energy of an incident gamma-photon is not competey absorbed by the Ge detector. The NaI(T) shied measures the escaping scattered photon, aowing simutaneous (Ge + NaI) Compton background events to be rejected. This produces significant suppression of the Compton background and a arge increase in the measured peak to background ratio.

16 Configuration Hardware gating for rea-time processing Utiise Canberra Lynx MCA with no dedicated coincidence eectronics Easiy configurabe and cost effective soution Additiona time-stamping functionaity Gate HPGe ICR NaI(T)

17 Improved sensitivity Produces a step-change in detection sensitivity Used by AWE for measurement of compex mixtures of radionucides Unsuppressed sampe Suppressed sampe The improved sensitivity offers potentia advantages for sampes containing mutipe interferences, incuding high yied fission products and their daughters such as Ba-140 (La-140) and Te-132 (I-132)

18 CTBT sampes Using SRID (suppression on/off) Compton suppression reduced background by 28 %. Suppression factors of increasing with energy. Improved MDA vaues typicay by 40 %. Probem True coincidence summing Cascade summing occurs when a singe nucear disintegration emits two or more photons and one is observed in the Ge detector. The NaI detector may measure the second photon escaping from the Ge crysta and reject the initia photon by Compton suppression. For any radionucide subject to summing, this sha produce a oss of measured counts and apparent reduction in detection efficiency. Appication for non-summing radionucides. Of the 85 CTBT radionucides, 52 are subject to true coincidence summing and are measured with reduced efficiency using the Compton suppression detector.

19 Fukushima incident Detection imits for Ba-140 reduced from 37 mbq to 25 mbq 134Cs 609 KeV gamma in cascade peak areas reduced ~ Counts 131I No cascade gamma s peak areas remain the same. Baseine reduced ~ Energy K 1000 Counts Counts Counts Energy KeV Energy KeV Energy KeV

20 PTE sampe 91Y Peak uncertainty unsuppressed Typica Compton Reduction Suppression factor of Suppressed 11.5% 3.4% Counts Energy (kev)

21 Monte-Caro Simuations Monte-Caro simuations rey upon repeated random samping to provide numerica resuts GEANT4/MCNPX are Monte-Caro tookits that simuate the passage of partices through matter Why? Better understanding of detectors Aows us to test unusua configurations & focus any future detector designs on our specific requirements Simuations are free, buiding test detectors is both expensive and time consuming How? Create simuations using Monte Caro package Test and vaidate these against existing detectors Use these modes to deveop new detector configurations A these are argey reiant on the nucear data ibraries upon which they are buit!

22 Exampe Simuations We can test scenarios that are inaccessibe in the aboratory X-ray reduction improved by a factor of 10 Compton Scattering of the source contributes 10x more events to the spectra than x-rays Cosmic muons increase the background rate as the amount of shieding increases however the overa effect is to reduce the background Britton R, Burnett JL, Davies AV, Regan PH (2013) JRNC, Vo 298-3, pp 1491

23 Simuating Compton Suppression To design a new system, Monte-Caro modes must be vaidated against existing systems GEANT4 modes were created for NaI(T) detectors Achieved agreement between data and simuation of greater than 91% from kev Britton R, Burnett JL, Davies AV, Regan PH (2012) JRNC, Vo 295-1, pp 573

24 Simuating Compton Suppression Moved onto HPGe crystas Used more advanced features of GEANT4 This was optimised and vaidated against experimenta data, achieving agreement >97% Britton R, Burnett JL, Davies AV, Regan PH (2012) JRNC, Vo 295-3, pp 2035

25 Simuating Compton Suppression Buit GEANT4 simuations of our Compton Suppression system Resuts show exceent agreement for a Broad-energy HPGE (BEGe) detector in conjunction with a NaI(T) guard detector Britton R, Burnett JL, Davies AV, Regan PH (2014) JRNC, Vo 300-3, pp 1253

26 Coincidence system corrections Cascade Summing corrections often have to be appied to radionucide abundance cacuations to account for summing out of summing in of each peak GEANT4 modes must aso reproduce this effect to be vaid It is aso crucia to be abe to mode this when simuating a fu Compton Suppression system Britton R, Burnett JL, Davies AV, Regan PH (2015) JRNC, Vo 299-1, pp 447

27 Coincidence system corrections Coincidence systems suffer from mutipe probems, incuding time-wak of ow-energy events and accidenta coincidences By coecting a data in ist-mode, it can be reprocessed to extract far more coincidence information than is typicay avaiabe Routines have been deveoped to correct for these effects Deay gate width reduced by 18.4% with timewak correction Effective remova of accidenta coincidences resuted in ~100% of 'accidenta' signas being removed Britton R, Burnett JL, Davies AV, Regan PH (2015) NIMA, Vo 769, pp 20

28 Coincidence system corrections Coincidence systems suffer from mutipe probems, incuding time-wak of ow-energy events and accidenta coincidences By coecting a data in ist-mode, it can be reprocessed to extract far more coincidence information than is typicay avaiabe Routines have been deveoped to correct for these effects Deay gate width reduced by 18.4% with timewak correction Effective remova of accidenta coincidences resuted in ~100% of 'accidenta' signas being removed Britton R, Burnett JL, Davies AV, Regan PH (2015) NIMA, Vo 769, pp 20

29 Improving the Current System Vaidated GEANT4 modes were utiised to design a new system Based upon od design, primary and secondary detectors were optimised Guard materia changed to BGO Additiona base guard detector to veto high energy gamma radiation that penetrates the system Resuted in substantia improvement in sensitivity for a range of nucides over current system (factor of 10) Primary detector - Britton R, Burnett J, Davies A, Regan P (2014) J Env Rad, Vo 134, pp 1 Secondary detector - Britton R, Burnett J, Davies A, Regan P (2014) NIMA, Vo 762, pp 42

30 Summary

31 Thank you