Scintillation detectors of ionizing radiation. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Dosimeters

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To detect ionizing radiation, they often use the ability of certain substances - scintillators - to make visible, luminous the trajectory of an ionizing particle "shooting through" them.

Scintillation detectors of ionizing radiation have a certain advantage over Geiger counters - the amplitude and duration of the flash can be used to judge the type and energy of the particle that generated it. It is also important that the scintillation counter has a much greater efficiency than the Geiger counter, which usually detects only one or two particles out of a hundred that fall into it.

Structurally, the scintillation counter is simple: the desired scintillator (see Appendix 7) is glued to the cathode of a photomultiplier tube (PMT) and all this is placed in a box carefully isolated from extraneous illumination. The rest is counting photopulses, sorting them by amplitude, shape, etc. - a matter of conventional electronic technology.

Schematic diagram of the photohead of the scintillation counter is shown in fig. 81, and a high-voltage converter to power it - in fig. 82. The PMT supply voltage - high with respect to the "ground" - is usually applied to its cathode. This makes it possible to galvanically connect the anode circuit of the PMT with the electronic analyzer of the device, taking into account, if necessary, the constant component of its photocurrent.

The PMT supply voltage, its distribution between the dynodes and, accordingly, the ratio of the values ​​of the resistors R2...R13 that make up the dynode divider depend on the type of photomultiplier (see Appendix 6). Here we have used a relatively low-voltage PMT-85. Since the PMT operation mode in household scintillators is close to "dark", the resistances of dynode resistors can be much higher than recommended (while maintaining the proportions).

Rice. 81. Photo head of a scintillation detector of ionizing radiation

The only operational adjustment in the channel - the resistor R14 - performs a very important function: on the comparator DA1 they are set to the threshold voltage U3-4. Only pulses with amplitude Uimp>U3-4 will open the comparator and a digital standard pulse will be generated at its output (pin 9).

In autonomous, dosimetric equipment using PMTs, the problem of their power supply arises. The high voltage UPM (0,8 ... 1 kV or more) required by the PMT, the requirements for its stability (the photosensitivity of the PMT quite strongly depends on the supply voltage; see Appendix 7) impose rather stringent requirements on devices that generate this voltage.

Rice. 82. PMT power converter

The basis of the high-voltage converter shown in fig. 82, constitutes a blocking generator that generates voltage pulses with amplitude Uimp on the winding II of the transformer T1@Ufeu. Through the diode column VD3 they charge the capacitor C5, which thus becomes the power source of the photomultiplier. Ufeu pulsations (they have the form of a "saw" with time intervals between the "teeth" tp@R7 C4) removes the RC filter (C5, R8, C6, R9, C7).

A transistor VT2 is introduced into the power circuit of the blocking generator, the collector current of which depends on the base current, which in turn depends on the drain current of the field effect transistor VT3. The gate voltage of this transistor depends on Ufeu, the voltage at the zener diode VD1 (transistor VT1 is its current-setting "resistor") and the ratio of the "shoulders" of the divider R3 + R4, R6 (resistor R3 sets the desired Ufeu). It is easy to see that if UPM decreases (in absolute value) due to some destabilizing reason, the supply voltage of the blocking generator will increase and the impact of the destabilizing factor will thereby be largely compensated.

The blocking generator transformer is wound on a ferrite ring M3000MN 20x12x6 mm. Due to the fact that this ferrite has a low volume resistance, the sharp edges of the core must be smoothed out and the entire core must be carefully insulated; wrap, for example, with lavsan or fluoroplastic tape.

Winding II is wound first, containing 800 turns of wire PEV-2 0,07. The winding is carried out in one direction, almost turn to turn, leaving a gap of 2 ... 3 mm between the beginning and end of the winding. Winding II is also covered with a layer of insulation. Winding I (8 turns PEVSHO 0,15 ... 0,25) and winding III (3 turns with the same wire) are laid on the core as evenly as possible.

The phasing of the windings (their common-mode ends are marked with dots on T1) must be observed when installing the transformer.

About the details of the converter. Resistor R6 - KIM-0,125, R3 - SP-38A, others - MLT-0,125 and 0,25. Capacitors C3, C4 - KM-6 or K10-176; C5, C7 - K15-5-N70 (1,5 kV) or other ceramic for a voltage of at least 1 kV; C1 and C2 - any oxide. The diode column 2Ts111A-1 can be replaced by four series-connected diodes of the KD102A type. For any other replacements, it must be borne in mind that the VD3 diode column must not only have a high reverse voltage - not less than UPM, but also a small (at this voltage) leakage current - no more than 0,1 μA.

The blocking generator transistor can be replaced with a KT630V. Here, the determining parameter is the saturation voltage of the transistor in the pulse mode: at a current in the pulse of 1 ... 1,5 A - Uke us imp Ј0,3 V. The residual voltage on the collector of the transistor can be easily estimated from the oscillogram: from the "gap" between the flat top of the pulse and the zero potential line.

The current drawn by the HV converter from the power supply will, of course, depend on the load. With the two scintillation heads described here, operating in the radiation radar mode, it did not exceed 16 mA.

Publication: cxem.net

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