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Introduction to
Charged-Particle Detectors
Alpha Resolution
Alpha resolution is specified as the maximum peak width for a standard
alpha source measured at one-half the peak height (FWHM) expressed in keV. The total
system alpha resolution is measured and warranted for 5.486-MeV alphas from
241Am
with an ORTEC preamplifier chosen to be consistent with the detector capacitance and an
ORTEC amplifier using equal differential and integral time constants as follows:
0.5 µs for A, B, C, F, L, and R Series;
1.0 µs for ULTRA, ULTRA-AS, and ULTRA CAM;
2.0 µs for P Series.
For totally depleted B Series detectors with £500-µm
thicknesses, the alpha resolution is measured through the exit (low-field strength)
contact, and >500-µm detectors measured with alpha particles through the front
contact. The D and F Series are not warranted for alpha resolution but are warranted for
system noise with suitable ORTEC electronics. Unless specified otherwise, resolution
measurements are performed and warranted at 21 ±1°C.Beta Resolution (System Noise)
The system noise width guaranteed maximum FWHM (which approximates beta
resolution) is listed for each type of detector. Unless specified otherwise, measurements
are performed and warranted at 21 ±1°C.Electrons (beta particles) are, to a first approximation, sufficiently
light to cause zero energy loss in the entrance window of silicon detectors. The beta
energy resolution of silicon detectors is thus
determined by the electronic noise of the detector and preamplifier; hence, the
interchangeable "beta resolution/system noise" terminology.
Discriminator Threshold SettingWhen silicon detectors are used as beta spectrometers, the threshold of
the lower-level discriminator in the electronics must be set at 2.5 times the "beta
resolution" to avoid spurious noise counts. Because the specific ionization of
electrons is very low (e.g., 0.35 keV/µm for minimum ionizing betas), it is often
necessary to cool silicon detectors used as beta spectrometers. (Detectors with cryogenic
epoxy must be special-ordered.) When this is done, the electronic noise caused by the
detector leakage current is eliminated and the detector becomes equivalent to a pure
capacitor. The electronic noise of the system can then be easily calculated from the noise
vs. capacitance characteristics of the preamplifier.
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