Activation detectors are materials which become radioactive when exposed to neutrons. Determination of neutron absorbed dose is made by measuring decay products of the activation foil (typically β- or γ-rays).
- A variety of materials are available allowing measurement of thermal and fast neutrons.
- Inexpensive allowing many to be used.
- Typically reusable after decay.
- Small allowing for collection of geometric information.
- Collect integrating information.
- Unwanted activation products can interfere with readout.
- No instantaneous readout.
- Requires additional counting equipment to read out.
- Readout must be performed prior to significant decay.
- Activation foil is placed in area with neutron presence for set period of time.
- Once exposure is complete, the foil must be taken to a sensitive detector to read out. This is typically done with thin window Geiger counter (for β- emitting foils) or a NaI Crystal or Gi(Li) detector (γ emitting foils).
- Measured foil activity is used to determine neutron fluence or dose.
Types of Activation Foils
Thermal Neutron Detectors
Operates by capturing neutron resulting in active daughter nuclide.
Most common detectors are Indium (In-116m, T1/2 = 54 minutes) and gold (Au-198, T1/2 = 2.7 days)
Threshold Activation Detectors
Detectors which require some minimum (threshold) energy to produce the desired reaction. Most often used to measure fast neutron flux.
The most common detector for fast neutron flux around accelerators is Sulfer-32 which has a 2.7MeV threshold for the 32S(n,p)32P reaction.
Moderated Foil Detectors
Another technique for measuring fast neutrons is to surround a thermal neutron detector with a moderator. This reduces the neutron’s energy allowing detection.
A polyethylene cylinder 15cm in diameter surrounding the foil is used to reduce the fast neutron energy. This cylinder is covered with 0.5-0.8mm of Cadmium which absorbs all incident thermal neutrons. In this way only fast neutrons are detected.