Current Integrator for Proton Dosimetry in Radiation-Chemical Studies

L. P. Eremin, V. F. Zaikin

(Presented by the scientific seminar of the “Cyclotron” facility of the Research Institute of Nuclear Physics, Electronics, and Automation)

In radiation-chemical studies, the accuracy of estimating the absorbed dose is of great importance. A reliable method of proton dosimetry is to determine the dose from the measured flux density of a proton beam, with the energy loss by the particles known [1].

Since the proton flux can vary substantially with time, it is expedient to measure it by means of integrating devices. The integrator circuits described in the literature [2] could not be used by us, since some of them are difficult to manufacture, while others do not satisfy the required parameters. The current-integrator circuit developed for our conditions, intended for measuring the proton flux, is based on converting the measured current of charged particles into a sequence of pulses using a one-shot multivibrator. The schematic diagram of the device is shown in Fig. 1.

The integrator is assembled on three tubes:
\(L_1\) — an electrometer tube; \(L_2\) — a one-shot multivibrator,
\(L_3\) — a cathode follower.

The negative charge of capacitor \(C_1\) is neutralized by the proton flux, causing an increase in the anode current of tube \(L_1\). At the same time the positive voltage on the grid of the left-hand triode (according to the circuit) of tube \(L_2\), taken from the divider \(R_2, R_3\), decreases.

In the normal state the left-hand triode of tube \(L_2\) is open. When the positive voltage on the left grid of tube \(L_2\) decreases, the anode current of the left-hand triode and the negative voltage on the grid of the right-hand triode decrease. The circuit parameters are chosen so that when the voltage on the grid of \(L_1\) rises to \(-3\) V, the right-hand triode of \(L_2\) opens and the multivibrator generates a pulse.

A positive pulse from the anode load of the left-hand triode \(L_1\), through capacitor \(C_2\), charges capacitor \(C_1\) by the grid currents of tube \(L_1\). By means of switch \(P_1\) it is possible to change the magnitude of the charge of capacitor \(C_1\) and, accordingly, the operating range of the integrator.

A negative pulse is taken from the anode of triode \(L_2\) and, through a cathode follower, is fed to the counting device. The current integrator is connected to the PST-100 counting instrument, from which a stabilized anode-supply voltage of \(+250\) V and 6.3 V for the tube filaments are supplied. The filament of tube \(L_1\) is powered by direct current, with the voltage stabilized by stabilovoltage tube \(D_1\).

The current being measured is fed from the detectors to capacitor \(C_1\) by a coaxial cable \(1\ \mathrm{m}\) long.

Fig. 1. Schematic circuit diagram of the current integrator

The circuit makes it possible to measure current in two ranges: from \(10^{-9}\) to \(10^{-7}\ \mathrm{A}\) and from \(10^{-8}\) to \(10^{-6}\ \mathrm{A}\); in this case the pulse repetition frequency varies over the interval from 20 to 2000 pulses/sec.

REFERENCES

1. A. N. Oblivantsev, L. P. Eremin, V. M. Lykhin. Teoretich. i eksperim. khimiya, 3, 844, 1967.

2. A. M. Bonch-Bruevich. Radio Electronics in Experimental Physics. Nauka Publishing House, Moscow, 621–651, 1966.