Detection of explosives and fissile material based on neutron generators, survey of techniques and methods. M. Bruggeman

Transcription

1 Detection of explosives and fissile material based on neutron generators, survey of techniques and methods M. Bruggeman TM neutron generators 1 Vienna, IAEA, June, 2005

2 Contents Context Initiating the FIMECS project at SCK Detection of fissile material Neutron/gamma counting techniques PGNAA Detection of explosives (shielded) Neutron backscattering PGNAA Imaging techniques Conclusions 2

3 Context = FIMECS What is FIMECS FIssile Material & Explosives Characterisation System Project to be initiated at SCK Investigate detection, analysis & simulation techniques for the detection/quantification of shielded fissile material and explosives in table-top experiments First actions: definition of geometry and detection requirements selection of n-generator (energy, n/s cm², APSTNG) selection of detectors selection of electronics, analysis methods 3

4 Detection of fissile material Physics Neutron induced fission, resulting in emission of: prompt neutrons (multiplicity distribution) prompt gamma-rays (multiplicity distribution) delayed neutrons delayed gamma-rays fission products (β-delayed γ-rays) spontaneous fission: neutrons/gamma rays U-235 does not emit significant neutrons, emits some low energy gammas Pu emit neutrons (even isotopes) 4

5 Detection of fissile material Methods Time Correlation methods n-generator (APSTNG) pairs time correlation between γ s and/or neutrons Rossi-alpha methods Multiplicity counting (at least 3 particles coincident in time) pulsed n-generator Differential die-away Counting of delayed neutrons Counting of delayed gamma-rays γ-spectroscopic prompt gamma-rays gamma-rays from fission products 5

6 Detection of fissile material (time correlation methods) Dx Detector (n & γ sensitive Dy Fissile Material Correlations (n-n, n-γ,ap) Dx Dy AP G n Generator Dx Dx Associated particle detector 14 MeV, 2.5 MeV 6 AP - Dx

7 Detection of fissile material (time correlation methods) BWXT-Y12/Bechtel Enterprise for DOE 7

8 Detection of fissile material (time correlation methods) source Oak Ridge Nat. Lab. 8

9 Detection of fissile material (time correlation methods) source Oak Ridge Y-12. 9

10 Detection of fissile material (time correlation methods) source Oak Ridge Nat. Lab. 10

11 Detection of fissile material (time correlation methods) Higher order correlations, Multiplicity counting involves the detection of 3 or more events, and the delay between the events GGN Ridge GGN & NNN GGG Peak source Oak Ridge Nat. Lab. 11

12 Detection of fissile material (via delayed high-energy γ- rays) Gross γ counting Method Detection of radiation signatures unique to SNM (delayed γ-rays from fission products) Counting the gamma rays > 3 MeV Interference with 16 O(n,p) 16 N is avoided by interrogation below 10 MeV (d-d generator) Pulsed neutron generator (collimated beam) Counting between pulses Decay time analysis (fission vs. Activation) Large volume detectors using scintillator coctail 12

13 Detection of fissile material (via delayed high-energy γ- rays) 16 N γ-rays Activation E < 2.5 MeV Characteristic half life 20 s 13

14 Detection of fissile material (time dependence of delayed neutrons) Detect neutrons after neutron burst of n- generator as a function of time n-decay is function of time constants of precursors (which is nuclide specific) Disadvantage: Low sensitivity due to low yield of delayed neutrons Low energy of delayed neutron ( kev) Large influence of shielding (degrading of performance) 14

15 Detection of fissile material (time dependence of delayed neutrons) Source Fraunhofer-INT 15

16 Detection of fissile material by PGNAA & short-lived NAA PGNAA 235 U E γ = 6397keV 238 U E γ = 4060keV Pu? Decay lines 16

17 Detection of high explosives Thermal neutron analysis Pulsed fast neutron analysis Pulsed fast-thermal neutron analysis APSTNG + TOF Neutron backscatter 17

18 Detection of high explosives (PGNAA) gamma spectrometry of capture gamma-rays. 18

19 Detection of high explosives (PGNAA) gamma spectrometry of capture gamma-rays MeV in nitrogen principal signature for explosives Neutron Inelastic Scattering Gamma Rays Measure the C:H:O concentration Neutron activation After a longer pause, Si, P 19

20 Detection of high explosives (PGNAA) gamma spectrometry of capture gamma-rays. hydrogen nitrogen 20

21 Fast neutron imaging IMAGING techniques E.g. MANDI (reflective scanning, transmission scanning) Associated Particle Imaging Neutron Tomography The use of portable neutron sources is being investigated (problem is beam collimation), Status and prospects of neutron tomography in Europe,

22 NMIS imaging by TOF IMAGING techniques Source: Oak Ridge Nat. Lab. 22

23 IMAGING techniques Reflective neutron imaging Source: Univ. of Rochester 23

24 IMAGING techniques Transmission neutron imaging Source: Univ. of Rochester 24

25 API API provides three-dimensional, color images of unknown materials hidden behind barriers Associated particle gives the direction (θ,φ) of the neutron and defines the emission time t=0 Arrival time t=t and energy of prompt gamma is recorded TOF defines the 3D co-ordinates where the interation occured The specific gamma-ray energy indicates the element 25

26 Conclusions Many physical methods have already been tested Advantages/disadvantages of one method to another are not always clear Most publications are always positive about the described methods, and there is always a signal, but Where to go from here? Methods relying on the detection of the modulation of primary radiation seems to be most promising (best s/n ratio) Imaging techniques are always a good choice When detecting secondary radiation s/n needs to be improved by API or nano pulse techniques Numerical simulation of experiments (MCNPX), model before 26 you buy/test