1. Backscatter neutron spectrometer

2. Rad Inter Neutrons W. Udo Schröder, 2011 2 Nuclear Interactions of Neutrons Characteristic secondary nuclear radiation/products:  -rays (n,  ) 2.charged particles (n,p), (n  ),… 3.neutrons (n,n’), (n,2n’),… 4.fission fragments (n,f) No electric charge  no direct atomic ionization Magnetic moment  interaction with magnetized materials Collisions and reactions with nuclei  10 -6 x weaker absorption than for charged particles Processes depend on available n energy E n : E n ~ 1/40 eV (= k B T) Slow diffusion, capture by nuclei E n < 10 MeV Elastic scattering, capture, nucl. excitation E n > 10 MeV Elastic+inel. scattering, various nuclear reactions, secondary charged reaction products

3. Rad Inter Neutrons W. Udo Schröder, 2011 3 Neutron Cross Sections 1b= 10 -24 cm 2 =100fm 2 n-hydrogen n-carbon

4. Rad Inter Neutrons W. Udo Schröder, 2011 4 Neutron Mean Free Path Mean Free Path of Neutrons in Water  : number density (atoms/ volume),  : cross section = average path length in medium between 2 collisions Los Alamos nuclear data files

5. Rad Inter Neutrons W. Udo Schröder, 2011 5 Neutron Diffusion In very thick absorbers, e.g., water tanks, concrete walls: Multiple scattering = statistical process Heavy materials (A>>1): random scattering Probability for no collision along path length x: P s (x) Probability for collisions at [x, x+dx]: dP coll =-dN/N(x)=-1/ f or = const.

6. Rad Inter Neutrons W. Udo Schröder, 2011 6 Energy Transfer in Elastic Scattering Neutron with lab velocity, energy E, scatters randomly off target nucleus of mass number A at rest in lab.  lab v cm|Lab vnvn v n|Lab vnvn vnvn  n Ac.m. v n v A lab velocity of center of gravity

7. Rad Inter Neutrons W. Udo Schröder, 2011 7 Scattered-Neutron Energy Spectrum Neutron with energy E 0 scatters off target nucleus A at rest in lab.  lab v cm|Lab vnvn v n|Lab vnvn vnvn  EnEn dP/dE n Lab energy spectrum (1 coll.) The laboratory energy spectrum of scattered n reflects the center- of-mass scattering angular distribution !  = 0 o  = 180 o

8. Rad Inter Neutrons W. Udo Schröder, 2011 8 Multiple n-A Scattering Incoming neutron energy E 0 N successive elastic scatterings Evaluate “Logarithmic Decrement”  N = 1 EiEi P(E n-A ) N = 2 N =3

9. Rad Inter Neutrons W. Udo Schröder, 2011 9 Thermalization Through Scattering A  N-therm 11.000018 120.1578115 650.0305597 2380.00842172 N-therm: E 0 =2 MeV  0.025eV

10. Rad Inter Neutrons W. Udo Schröder, 2011 10 Slow-Neutron Resonance Capture n- 107 Ag  108 Ag* Quantal resonance effect at low and thermal n energies  wave nature of neutrons Excited Ag* nucleus deexcites and/or  - decays

11. Rad Inter Neutrons W. Udo Schröder, 2011 11 n-Stopping and Scintillation Process in Thick Detector e-e- e-e- 10 -7 s 10 -6 s10 -5 s10 -4 s 10 -8 s diffusion delayed@  8MeV  m n interactions Gd      e-e- e-e- delayed delayed light Fast Moderation Process Organic Scintillator Diffusion Regime time time=0 Gd n1n1 n2n2 n3n3 Prompt Injection of m neutrons

12. SuperBall Neutron Calorimeter

13. Rad Inter Neutrons W. Udo Schröder, 2011 13

14. Rad Inter Neutrons W. Udo Schröder, 2011 14 Pulse Shape Analysis fast component slow I t2t2 t1t1 t0t0 Particle 1 Particle 2 2 signals of equal total light output Q 1 -Q 2

15. Total n-H Cross Section Parameterization Approximate parameterization for applications (detector efficiency estimates) Cross section in barns (b) 1b = 10 -24 cm 2

16. Rad Inter Neutrons W. Udo Schröder, 2011 16 Non-Linear Light Output Response NE 213 liquid scintillator: e-equivalent energies E e  E p For a given energy, electrons & photons have the highest light output

17. Thin-Detector Light Output Response Rad Inter Neutrons W. Udo Schröder, 2011 17 Thin detector: 1 n-p interaction within detector (n leaves) Equivalent to -e Compton Each fixed neutron energy produces recoil proton energy (E p ) distribution  produces distribution in LO (light output). LO response is calibrated with -rays  electron- equivalent energy E ee (eV ee ). Convert measured E ee  E p Unfold E p distribution 1 Light Output Response 5”x2” NE-213 1.5 2.0 Neutron Energy 2.1 2.5 3.0 MeV Equivalent to full-energy peak?  Thick detector (many thick)

18. Rad Inter Neutrons W. Udo Schröder, 2011 18 Efficiency of p-Recoil NeutronDetectors EnEn dP/dE p  = 0 o E p =E n ETET F1F1 F2F2 5”x2” NE-213 Electronic detector threshold ETET angle dependent n-p energy transfer  continuous recoil energy spectrum

19. Detector Efficiency Estimates Detector thickness d =10cm Hydrogen density (atoms/cm 3 ) NE-213 n H =4.86·10 22 /cm 3 E th =1 MeV Approximate intrinsic detection efficiency  (E) Probability for detection if incident neutron trajectory is perpendicular to detector face. Total efficiency contains  (E) and solid-angle factor . Rad Inter Neutrons W. Udo Schröder, 2011 19

20. Associated-Particle/Neutron TOF Rad Inter Neutrons W. Udo Schröder, 2011 20 Reference time-zero t 0 : true TOF = t meas -t 0 a) from accelerator signal, b) associated particle* of known E c)  -ray* (v=c) *measured with same or different detector d= target-detector flight distance d tt

21. Rad Inter Neutrons W. Udo Schröder, 2011 21

22. Rad Inter Neutrons W. Udo Schröder, 2011 22 n Angular Distribution  lab v cm|Lab vnvn v n|Lab vnvn  > 0  forward scattering vnvn

23. Rad Inter Neutrons W. Udo Schröder, 2011 23 A Dependence of Angular Distribution Properties of n scattering depends on the sample mass number A  Measure time-correlated flux of transmitted or reflected neutrons  lab Light Nucleus Heavy Nucleus Light nuclei: slowing-down and diffusion of neutron flux Heavy nuclei: neutrons lose less energy, high reflection & transmission

24. Rad Inter Neutrons W. Udo Schröder, 2011 24 Principle of Fast-Neutron Radiography(Imaging)       (  ) incoming transmitted neutrons Transmission decreases exponentially (reflectivity increases) with thickness and density of sample Transmission decreases exponentially (reflectivity increases) with thickness and density of sample. Neutrons more penetrating  use for thick samples sample Comparison of different radiations

25. Rad Inter Neutrons W. Udo Schröder, 2011 25 Commercial Neutron Generator ING-03 VNIIA Source: All-Russian Research Institute of Automatics VNIIA 1300 mm rear connectors source window Specs: < 3·10 10 D(d,n) 3 He neutrons/s Total yield 2·10 16 neutrons Pulse frequency 1-100Hz Pulse width > 0.8  s, Power 500 W Alternative option: T(d,n) 4 He, E n  14.5 MeV Neutrons can be produced in a variety of reactions, e.g., in nuclear fission reactors or by the D(d,n) 3 He or T(d,n) 4 He reactions

26. Rad Inter Neutrons W. Udo Schröder, 2011 26 MANDI-01 A Mobile Accelerator-Based Neutron Diagnostic Imager

27. Rad Inter Neutrons W. Udo Schröder, 2011 27 Neutron interactions with sample nuclei may produce characteristic secondary radiation:  -rays (n,  ) 2.charged particles (n,  ),… 3.neutrons (n,n’) 4.fission fragments (n,f) depending on the sample material Principle of Fast-Neutron Imaging (3) time intensity primary neutrons secondary radiation Secondary radiation induced by neutrons in the sample appear with the same frequency as the neutron pulses. Detectors for characteristic secondary radiation improve recognition of sample material, reduce ambiguities.

28. Rad Inter Neutrons W. Udo Schröder, 2011 28 Required R&D Design and construct MANDI test mounts & hardware Measure n energy spectra and angular distributions with and without different types of collimators. Design and test B-loaded plastic shielding/moderator. Perform extensive pulsed-beam coincidence measurements of 2.5-MeV n transport through a range of materials varying in density and spatial dimensions. Measure n amplification in thick fissile targets Assess sensitivity and quality of transmission and backscattering imaging Develop computer model simulations Develop large-area detectors (e.g., BF 3 or BC454 B- loaded scintillation counters) Develop and test dedicated electronics Stage I Stage II