IEC 60462 pdf download – Nuclear instrumentation – Photomultiplier tubes for scintillation counting – Test procedures

IEC 60462 pdf download – Nuclear instrumentation – Photomultiplier tubes for scintillation counting – Test procedures
4 Test conditions
Test conditions for photomultipliers are specified in terms of environmental conditions that shall be met to enable accurate measurements of the photomultiplier parameters discussed in this standard. Power supplies should be stabilized and, in particular, high-voltage power supplies should have regulations of 0,01 % or better, and ripple and noise should be not more than 1 0 mV pp .
The test enclosure shall be free of detectable light leaks. This can be verified by half-hour photon counting periods, with and without bright ambient light incident on the enclosure. The PMT should be stored in darkness for 1 h prior to measurement to avoid phosphorescence effects.
Cleanliness of the PMT glass and sockets is essential in preventing external noise effects. Any material near the photocathode should be at photocathode potential to prevent electro-luminescence of the envelope and electrolysis or charge accumulation of the glass. To obtain the best conditions for reproducibility of tests, it is recommended that where feasible, a shield connected to cathode potential, be placed around and in contact with the glass envelope of the photomultiplier.
The PMT should be degaussed before using, and a magnetic shield should be employed. Note that even the earth’s magnetic field is of sufficient strength to influence measurements.
Tube temperature should preferably be maintained constant at ± 2 °C within the limits from 1 9 °C to 25 °C. This is important in instances where the voltage divider may raise the temperature of the test enclosure. Caution should be used to avoid drifts or base line shifts in the electronic circuitry that significantly affects the measurements. To prevent drifts or base line shifts in potentials between dynodes resulting from the electron multiplier current, the quiescent current drawn by the resistive voltage divider should be at least 20 times the DC anode current.Alternatively, the potentials between dynodes for thedynodes drawing the greatest current may be individually stabilized (as with separate powersupplies).
Charge-storage capacitors may be effectively used across the dynodes or from the dynodesto ground when the ratio of the peak anode current to the average anode current is large andthe capacitor can maintain the required dynode potentials for the duration of the pulse.
Pulse shaping methods and time constants suitable for optimum performance should be usedand should be stated.
5 Test procedures for photomultiplier characteristics
5.1General
ln addition to the specifications and test methods of lEC 60306-4, complementary or extendedspecifications and tests required for photomultipliers used with scintillation and Cherenkovdetectors are:
a)Pulse height characteristics1.b)Dark current.
)Pulse timing characteristics.5.2Pulse height characteristics5.2.1General
Pulse height is used in counting and spectrometric applications.
5.2.2Pulse height resolution measurement
5.2.2.1General
In general there are four distinct PHR measurements to define the photon-and-electronresolution of PMT and scintillator/PMT combinations. These resolutions may be usedseparately or together.
5.2.2.2137cs PHR for a scintillator/PMT combination
This PHR is a function of the photocathode quantum efficiency, collection efficiencies of thedynodes and spatial uniformity, as well as the resolution of the scintillator.
For standard cases,measurement of 13Cs pulse height resolution requires a 13’Cs source,aNal(TI) scintillator of 50 mm height and approximately the same diameter as thephotocathode,a pulse height analyzer and the photomultiplier to be tested. Thephotomultiplier tube is optically coupled to the scintillator – for example,with the aid ofsilicone grease or viscous oil. The crystal housing should be at photocathode potential. Thesource is placed at a distance from the scintillator such that less than 1 000 pulses/s areencountered.
The PMT should be operated at a voltage such that a linear response is obtained,i.e. theoutput pulse height is proportional to input light intensity. lmproper anode bias,excessive gain(and thus excessive anode current) or improper voltage divider circuits may give rise to acompression of the output pulse distribution,yielding an incorrect (low) value of PHR.
The tubelscintillator combination should warm-up for 1 h to obtain optimum PHR.