Interactions of Neutrons with Matter

Commercial Neutron Generator ING-03 Neutrons can be produced in a variety of reactions, e.g., in nuclear fission reactors or by the D(d,n)3He or T(d,n)4He reactions 1300 mm rear connectors source window Specs: < 3·1010 D(d,n)3He neutrons/s Total yield 2·1016 neutrons Pulse frequency 1-100Hz Pulse width > 0.8 ms, Power 500 W Rad Inter Neutrons Alternative option: T(d,n)4He, En15 MeV Source: All-Russian Research Institute of Automatics VNIIA W. Udo Schröder, 2003

Nuclear Interactions of Neutrons No electric charge  no direct atomic ionization  only collisions and reactions with nuclei  10-6 x weaker absorption than charged particles Processes depend on available n energy En: En ~ 1/40 eV (= kBT) Slow diffusion, capture by nuclei En < 10 MeV Elastic scattering, capture, nucl. excitation En > 10 MeV Elastic+inel. scattering, various nuclear reactions, secondary charged reaction products Characteristic secondary nuclear radiation/products: g-rays (n, g) charged particles (n,p), (n, a),… neutrons (n,n’), (n,2n’),… fission fragments (n,f) Rad Inter Neutrons W. Udo Schröder, 2003

Neutron Cross Sections 1b=10-24cm2=100fm2 Rad Inter Neutrons W. Udo Schröder, 2003

Mean Free Path of Neutrons in Water Neutron Mean Free Path 35 30 25 20 15 10 5 4 6 8 10 12 14 Neutron Energy (MeV) Neutron Mean Free Path (cm) Mean Free Path of Neutrons in Water r: atomic density s: cross section l = average path length in medium between 2 collisions Rad Inter Neutrons W. Udo Schröder, 2003

Neutron Resonance Capture n-107Ag  108Ag* Quantal resonance effect at low and thermal n energies  wave nature of neutrons Rad Inter Neutrons W. Udo Schröder, 2003

Energy Transfer in Elastic Scattering qlab Neutron with lab velocity , energy E, scatters randomly off target nucleus of mass number A at rest in lab. n A c.m. vn vA qlab vcm|Lab vn vn|Lab q lab velocity of center of gravity Rad Inter Neutrons W. Udo Schröder, 2003

Neutron Energy Spectrum Neutron with energy E0 scatters off target nucleus A at rest in lab. Lab energy spectrum (1 coll.) En dP/dEn-A qlab vcm|Lab vn vn|Lab q q = 180o q = 0o The laboratory energy spectrum of scattered n reflects the center-of-mass scattering angular distribution ! Rad Inter Neutrons W. Udo Schröder, 2003

Multiple n-A Scattering Ei P(En-A) N = 2 N =3 <E1> <E2> <E3> Incoming n energy E0 = < E0>, N successive scattering events “Logarithmic Decrement” x Rad Inter Neutrons W. Udo Schröder, 2003

Thermalization Through Scattering N-therm: E0=2 MeV  0.025eV A x N-therm 1 1.0000 18 12 0.1578 115 65 0.0305 597 238 0.0084 2172 Rad Inter Neutrons W. Udo Schröder, 2003

n Angular Distribution qlab vcm|Lab vn vn|Lab q vn Rad Inter Neutrons > 0  forward scattering W. Udo Schröder, 2003

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 Light Nucleus qlab Heavy Nucleus qlab Rad Inter Neutrons Light nuclei: slowing-down and diffusion of neutron flux Heavy nuclei: neutrons lose less energy, high reflection/transmission W. Udo Schröder, 2003

Principle of Fast-Neutron Imaging (2) neutrons neutrons sample f(0)  d  f(d) incoming transmitted Rad Inter Neutrons Transmission decreases exponentially (reflectivity increases) with thickness and density of sample, for most materials. W. Udo Schröder, 2003

Number of collisions E0  E1 Neutron Diffusion Multiple scattering = statistical process Heavy materials (A>>1): random scattering Number of collisions E0  E1 Probability for no collision along path length x: P(x) for l = const. Rad Inter Neutrons W. Udo Schröder, 2003

Mean Free Path of Neutrons in Water Neutron Mean Free Path 35 30 25 20 15 10 5 4 6 8 10 12 14 Neutron Energy (MeV) Neutron Mean Free Path (cm) Mean Free Path of Neutrons in Water r: atomic density s: cross section l = average path length in medium between 2 collisions Rad Inter Neutrons W. Udo Schröder, 2003

n-Stopping and Scintillation Process Gd g e- delayed delayed light Organic Scintillator Liquid Diffusion Regime e- e- Prompt Injection of mn neutrons 10-8s 10-7s 10-6s 10-5s 10-4s Rad Inter Neutrons diffusion delayed@8MeV mn interactions time time=0 W. Udo Schröder, 2003

A Mobile Accelerator-Based Neutron Diagnostic Imager MANDI-01 Rad Inter Neutrons W. Udo Schröder, 2003

Principle of Fast-Neutron Imaging (3) time intensity primary neutrons secondary radiation Neutron interactions with sample nuclei may produce characteristic secondary radiation: g-rays (n, g) charged particles (n, a),… neutrons (n,n’) fission fragments (n,f) depending on the sample material Secondary radiation induced by neutrons in the sample appear with the same frequency as the neutron pulses. Rad Inter Neutrons Detectors for characteristic secondary radiation improve recognition of sample material, reduce ambiguities. W. Udo Schröder, 2003

Design and construct MANDI test mounts & hardware 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., BF3 or BC454 B-loaded scintillation counters) Develop and test dedicated electronics Stage I Rad Inter Neutrons Stage II W. Udo Schröder, 2003