Neutron Detection

REM sphere

polyethylene sphere with cadmium that can detect neutrons

Bonner sphere

LiI scintillator placed at center of polyethylene moderating spheres of different diameters

Fast neutrons

> 1 keV

Slow neutrons

< 1 eV

Thermal neutrons

0.025 eV (2200 m/s)

Neutron cross section

likelihood of interaction bt nucleus and neutron

Neutron dose equivalent

impact of radiation on human tissue

Physical principles inherent to thermal neutron detection

neutron capture and ionization

Thermal neutron reaction with 10B and 6Li

Li and alpha take turns interacting with wall; 6% of time produces Li in ground state; 94% of time produces excited Li; gamma is 48 keV

Differential pulse height spectrum of BF3 proportional counter

Relationship between dose response of REM sphere and neutron dose equivalent as a function of neutron energy

If E fixed, as radius increases it hits a max then declines; if R is fixed, as E incs it hits a max then declines

Primary fast and thermal neutron interactions in tissue

interactions and weighting factor depend on tissue type and charged particles produced

3 means of fast neutron detection

1. Counters based on neutron moderation

2. Detectors based on fast neutron-induced reactions

3. Detectors utilizing fast neutron scattering

Neutron shield

Neutron source then mediator (like water) then absorber (like Cd-13), then person