Delayed Particles in Showers

In the paper under review*) the time distribution of delayed particles in extensive air showers at an altitude of 130 m above sea level was investigated. The showers were recorded (see Fig. 2) by two trays \(B\) and \(C\) of Geiger–Müller counters with an area of \(1260\ \text{cm}^2\), and by a scintillation

) J. V. Jelly and W. J. Whitehouse, Proc. Phys. Soc. A, 66*, 454 (1953).

with counter \(A\), having a large surface area and considerable volume, arranged in a horizontal plane at the angles of a triangle with sides of 5 m.

The use in these experiments of a scintillation counter makes it possible to avoid a number of difficulties that arise when carrying out such work with Geiger counters. Among such difficulties is the delay in the onset of the discharge in the counter relative to the moment when the particle passes through the counter. Fluctuations in the delay time limit the accuracy of measuring time intervals to a few units of \(10^{-7}\) sec. In addition, the presence of fluctuations also leads to considerable errors in measuring the distribution of pulses in time, since the number of registered pulses is usually small.

Fig. 1.

Geiger–Müller counters have a rather large dead time (\(\sim 10^{-4}\) sec), and therefore, after the passage of a shower, a significant fraction of the counters in the arrays are insensitive to delayed particles, which in turn leads to a deterioration of the statistical data.

In the work under consideration, a large scintillation counter was used to register showers and particles delayed in them. A schematic drawing of the counter is shown in Fig. 1.

A scintillating solution of p-terphenyl in pure benzene (2 g/l) was poured into a cylindrical brass container with silvered walls. The area of the bottom of the container was \(1000\ \text{cm}^2\), the depth was 15 cm. The top of the container was closed with glass 6.5 mm thick. The reflector was made in the form of a truncated cone, the inner surface of which was coated with MgO. An EM1-5311 type multiplier together with a preamplifier was mounted on the casing that closed the counter from above.

Preliminary experiments showed that with a discriminator bias of \(0\)—12 scintillation counters had an efficiency of \(98 \pm 2\%\) for particles passing through the central part of the scintillator, and \(91 \pm 6\%\) for particles passing near its edges.

The block diagram of the entire apparatus is shown in Fig. 2.

Pulses from the chambers from counters \(B\), \(C\), and from the scintillation counter \(A\), through the corresponding amplifiers, were fed to the triple-coincidence circuit \(D\) (\(\tau = 0.75\ \mu\mathrm{sec}\)), and the resulting pulse was used to start the waiting sweep of the oscilloscope. The duration of the sweep was \(1\ \mu\mathrm{sec}\); the time marks were supplied by a \(10\ \mathrm{MHz}\) generator.

Pulses from the scintillation counter were fed to the vertical plates of the oscilloscope through a \(0.3\ \mu\mathrm{sec}\) delay line, \(E\), and a traveling-wave amplifier \(G\) (23 stages, bandwidth—\(100\ \mathrm{MHz}\), gain 200).

Fig. 2.

When a wide shower of cosmic particles passed through, the coincidence circuit operated, starting the sweep, and the pulses both from the non-delayed and from the delayed particles were recorded photographically on the screen of oscilloscope \(H\).

Measurement of the pulse amplitudes and of the time intervals between the delayed and non-delayed pulses was carried out with the aid of a microprojector with a magnification of 12.5 times. The apparatus provided for continuous recording of the counting rate of the scintillation counter and of the coincidence circuit.

The study of delayed particles was carried out in the range \((3 \div 70)\times 10^{-8}\ \mathrm{sec}\). The authors estimate the resolving time of the apparatus as \(3\cdot 10^{-8}\ \mathrm{sec}\). In all, during the course of the work, 55,500 showers were registered, of which 322 showers were accompanied by delayed particles.

The resulting experimental distribution can be represented by an exponential function. Half of the delayed particles fall within the interval \((10 \pm 2)\cdot 10^{-8}\ \mathrm{sec}\). As the authors point out, in

In the above-mentioned range, \(0.85 \pm 0.05\%\) of shower particles are delayed; moreover, the particles accompanied by delayed particles do not differ in mean intensity from showers that do not contain delayed particles. No relation was established between the amplitude of the delayed pulses and the delay time.

—Yu. Sh.