BS ISO 14850-1 pdf download Nuclear energy — Waste-packages activity measurement — Part 1: High-resolution gamma spectrometry in integral mode with open geometry
4 Detectors and ancillary equipment
The measuring station usually comprises the following equipment.
a) Mechanical equipment:
a package-positioning system (rotation, with optional vertical movement);
a detector-positioning system (vertical, horizontal and distance);
a weighing station (optional);
collimator, background shielding, gamma ray attenuators,
b) Detector and signal-processing electronics:
a detector and preamplifier;
an analog-digital converter,
a “stand alone” module or a computer interface card,
c) Computer with measurement processing and interpretation software.
4.1 Mechanical equipment
4.1.1 Package-positioning system
The package-positioning system is designed to support the package to be measured, and to move it in rotation about its vertical axis. It may also allow for vertical moving of the package, and may be equipped with a weighing system.
4.1.2 Detector-positioning system
This system carries the detector with its protective shielding, and positions it in elevation and azimuth with respect to the package to be measured. It may be equipped with a system to determine the relative position of the package with respect to the detector.
4.2 Detector and preamplifier
The method covers only high-purity germanium semiconductor detectors. Two types of detectors may be selected depending on the energy of the radionuclides to be measured:
planar or flat coaxial detectors provide better resolution at low energy (below 400 keV),coaxial detectors give higher efficiency at high energies.
The semiconductor crystal requires a cryogenic system. The detector signal is collected by a charge sensitive preamplifier; this can be of either the resistive feedback type, transistor reset type or pulsed optical feedback type depending upon the application.
The amplifier implements Gaussian or triangular pulse shaping with a time constant adjustable from 0,25 µs to 1 5 µs. A pileup rejector is generally used, and in some cases the amplifier is equipped with a gated integrator.
Various types of amplifiers may be used in conjunction with this method. The choice of an amplifier depends on the other components in the counting system.
4.4 Analog-digital converter (ADC)
Two types of ADC converters are used in gamma spectrometry:
Wilkinson ADCs, with a variable dead time; counting losses depend on the conversion frequency and the signal amplitude;
successive approximation ADCs, with a fixed dead time independent of the signal amplitude.
4.5 Multi-channel analyzer (MCA) and data processing system
The analyzer stores the encoded data in a basic memory array available to the computer.
NOTE A digital signal processing module may replace the functions described in 4.2, 4.3 and 4.4. It quantizes the preamplifier output signal, allowing higher counting rates
4.6 Background shielding
The unit is shielded against external radiation by (preferably low-activity) lead or other high-density materials.
The dimensions depend on the characteristics of the measurement environment.