Hi-pSi Semiconductor Detectors
for Radiation Field Scanning
Semiconductor detectors offer many practical advantages
for radiation field analysis. They are small, mechanically stable
devices which deliver a high level signal output. Bias voltages
are not required, and the response is very fast and independent
of pressure variations. Their small size allows the detection
area to be located very close to the surface (0.5mm) and
provides excellent spatial resolution. The sensitivity is
extremely high – typically about 18,000 times that of an air-filled
ionization chamber of the same volume.
The waterproof Hi-pSi semiconductor radiation field detectors are designed to meet the diverse needs of photon and electron radiation field analysis. They are delivered preirradiated, which stabilizes sensitivity at the expense of a slight temperature coefficient. However, for extensive water phantom measurements, detector stability with accumulated dose is a much more important consideration than variation due to temperature change.
Model EFD-3G Electron Field Detector
The water/silicon mass collision stopping power of the 3G-pSi semiconductor detector is practically constant at energy levels above 5MeV. This allows depth-ionization curves for electron beam measurements to be used as depth-dose curves without requiring correction factors. Electron beam measurements may be made as close as 0.5mm to the phantom surface, and directional dependence is less than 2.5% for a 20 x 20cm field at 2cm depth in a 20MeV beam.
Model PFD-3G Photon Field Detector
In photon beams, depth-dose percentages obtained from a semiconductor tend to be overestimated. This tendency increases with the depth due to increased sensitivity of the semiconductor to energies below 400keV. To prevent this, the Hi-pSi semiconductor chip is backed with a tungsten/epoxy shield to capture low-energy back-scattered photons. The result is an energy-compensated photon detector giving precise readings from 50 to 400mm.
Model SFD-3G Small Field Detector
The very small size of the stereotactic detector, its energy independence in fields less than 10cm, and the low impact of non-electron equilibrium makes it the obvious choice for measuring output factors and depth-dose curves in stereotactic beams.
Model RFD-3G Reference Field Detector
This is a relatively low-cost detector that should only be used to obtain a reference signal for relative field measurements. By calculating the quotient of the field and reference signals, the effect of variations in dose rate from the accelerator are nulled out. It is best not to use an ion chamber as the reference when the field detector is a diode because of signal size and the need for bias.