METHOD FOR MEASURING AND SELECTING THE TIME CHARACTERISTICS OF SCINTILLATION COUNTERS AND ELECTRONIC CIRCUITS

I. V. GLAVANAKOV, V. I. KRYSHKIN, V. N. STIBUNOV, N. P. FEDOROV

(Presented by the Scientific and Technical Seminar of the Laboratory of High Energies, Nuclear Physics Research Institute)

The most important requirement for recording systems (a set of scintillation, Cherenkov, and other counters and electronic circuits) is high efficiency in recording particles of the process under study. Under conditions of large background loads, this requirement leads to the need to have the minimum time resolution of the elements of the recording system. This paper describes a method for measuring and selecting the optimum characteristics of counters and electronic circuits in order to obtain high recording efficiency. The method was used in tuning a recording system consisting of two telescopes (5 scintillation and Cherenkov counters).

I. Selection and measurement of counter characteristics

Since the time resolution of counters is determined by the characteristics of the photomultiplier tubes used (PMTs) (for small sizes of the plastic scintillator), the operating mode of the PMT supply was selected by the usual method [1] using a GaP light-emitting diode. The final selection of the PMT and its supply mode was carried out under the following conditions: directly on the ends of a plastic scintillator 5 cm in diameter and 1.5 cm thick, irradiated by a radioactive source Co$^{60}$, two PMTs were mounted. Pulses from the anodes of the PMTs were shaped and fed to a time-amplitude converter (TAC) without additional selection of pulses by amplitude in the “side” channels. In this case the discriminator levels of the shapers had a value corresponding to the recording of electrons with an energy of 0.1–0.2 MeV. The time resolution of the best FEU-30 specimens, measured under these conditions, was $2\tau = 0.65$–$0.7$ ns. The time resolution was measured on a divergent beam of relativistic electrons, providing uniform irradiation of the entire area of the scintillators. Table 1 gives the results of these measurements. As the “reference” counter, a counter with scintillator dimensions $10 \times 8 \times 0.7$ cm$^3$ was used. Measurements for counter No. 4 were carried out using two TACs to compensate for the change in the delay of the PMT pulse depending on the point at which the electron passed through the scintillator [2].

Table 1

2. Setting the Discrimination Levels of the Shapers

Pulses from the PMTs were fed to shapers with compensation of the amplitude dependence, based on the well-known “following-threshold” principle. The circuit implementation of this method is analogous to that proposed in [3], i.e., summation of the inverted and “clipped” PMT signal at a certain level with the non-inverted PMT signal delayed by the rise time. The amount of delay of the non-inverted signal was selected for each type of PMT. The change in the delay of signals with amplitude exceeding \(3U_{\text{p}}\) (\(U_{\text{p}}\) is the shaping threshold level) at the shaper output does not exceed 0.1 ns.

In measurements of the time resolution of counters, the supply voltage was selected so as to ensure minimal time fluctuations. After this, the apparatus efficiency of registration of the counter at the selected voltage was checked. A block diagram for measuring the apparatus efficiency of a counter with a shaper is shown in Fig. 1. The signal from the PMT anode is split in a linear

In the diagram: Counter; L.S.; L.G.; I.; A.A.; Sh.; D.T.S.

Fig. 1. Block diagram of the setup for setting the discrimination levels of the shapers.

splitter (LS) into two signals, one of which is fed to the input of the linear gate (LG), and the other to the shaper under study (Sh.), which controls nanosecond linear gates. To avoid frequency overloads of the spectrometric amplifier and the amplitude analyzer (AA), control of the linear gates was carried out by a dead-time shaper (DTS) \((\tau_{\text{dead}} = 50 \div 100\ \mu\text{s})\). The signal from the output of the linear gates was stretched without amplitude distortion and fed to the AI-256 amplitude analyzer. In this case the analyzer records only those pulses whose amplitude exceeds the discrimination level of the shaper. The efficiency of the counter with the shaper is easily determined from the form of the amplitude distribution. These measurements make it possible to dispense with studying the counting characteristics.

All circuits of the spectrometric channel and their characteristics are described in [4].

3. Measurement and selection of telescope characteristics

Usually, measurements of the instrumental efficiency of a telescope, the number of random coincidences, the selection of particles with a specified velocity, and the time matching of telescope elements are carried out by the delayed-coincidence method [5] successively for each of the circuits. Such measurements require monitoring of the intensity and knowledge of the “time” structure of the beam passing through the telescope. When the characteristics of the telescope elements are adjusted, the measurements are repeated and require large expenditures of time.

In the present work, the measurement and time matching of telescope elements and of telescopes is carried out by means of a time-to-amplitude converter [6], on the basis of the method proposed in [7] for measuring curves of random coincidences and anticoincidences. The principle is explained with a simple example of a telescope element consisting of two counters and a coincidence circuit. A block diagram of the measurements is presented in Fig. 2. Figure 3a shows the time-of-flight spectrum from these counters when the pulses were fed directly to the TAC, without a coincidence circuit. The peak is associated with relativistic particles with \(\beta \approx 1\) (electrons, etc.).

Fig. 2. Block diagram for measuring the characteristics of a telescope consisting of two counters.

In Fig. 3b the same spectrum is shown, but the pulses from the shapers were fed to the TAC through “gates” controlled by a coincidence circuit with a resolving time greater than the width of the electron peak at the base, so that the coincidence circuit registered all electrons. The resolving time of the other coincidence circuit (Fig. 3c) is smaller, but its efficiency is 100%, since the width of the electron peak at the base is equal to the resolving time of this circuit; in this case it is minimal for effective selection of electrons. Figure 3d shows the spectrum when the “gates” are controlled by a coincidence circuit with a resolving time smaller than the width of the electron peak at the base and, consequently, with an efficiency less than 100%. The exact value of the efficiency is easily determined from the ratio of the areas under the peaks normalized in height in Figs. 3d and 3c. For comparison, Fig. 3e shows the delayed-coincidence curve of the same coincidence circuit as in Fig. 3c. The difference in the shape of the curves is related to the magnitude of the resolving time of this circuit.

Usually the efficiency of a coincidence (or anticoincidence) circuit is judged by the presence of a plateau on the delayed-coincidence curve, which, at the minimum resolving time, will be absent (Fig. 3c). To suppress the background of random coincidences, it is often necessary to determine the smallest resolving time for effective registration of the events under study. This method makes it possible to do this quickly and clearly. In addition, from the height of the pedestal on which

Fig. 3, a–d. Time distribution of pulses from two counters, recorded by a coincidence circuit for different resolving times; e — delayed-coincidence curve.

the peak of correlated events is located, the contribution of accidental coincidences can be determined. Note that the method for estimating accidental coincidences by introducing a delay into one of the channels assumes a uniform distribution of background events, which is generally not true.

References

1. V. I. Rykalin, T. G. Kmita, I. V. Ryzhikov, I. A. Novoselov. JINR Preprint, P-2466, Dubna, 1965.

2. V. T. Lapshin, V. I. Rykalin, Z. Tsisek. JINR Preprint, No. 13-3530, Dubna, 1967.

3. M. Drazhev, S. Parzhitskii. JINR Preprint, No. 13-4604, Dubna, 1970.

4. V. A. Aulchenko, Yu. V. Korshunov. Preprint, Institute of Nuclear Physics, Siberian Branch of the USSR Academy of Sciences, 14, Novosibirsk, 1970.

5. Yu. K. Akimov. Scintillation Methods for Recording High-Energy Particles. Moscow, Moscow State University Press, 1963.

6. V. I. Kryshkin, V. N. Stibunov, N. P. Fedorov. PTE, No. 6, 66, 1970.

7. V. I. Kryshkin, V. N. Stibunov, N. P. Fedorov. PTE, No. 4, 99, 1971.