FROM THE CURRENT LITERATURE

“MONOCHROMATIZATION” OF A BEAM OF $\gamma$ QUANTA AT A SYNCHROTRON

As is well known, the principal source of high-energy $\gamma$ quanta is the radiation of electrons decelerated in the target of resonance accelerators. The circumstance that the spectrum of bremsstrahlung has a continuous character often makes it difficult to interpret physically the results of studying various photonuclear reactions, since in each individual case (with the exception of photodisintegration of the deuteron and some other reactions) the energy of the $\gamma$ quantum cannot be determined directly from experiment.

Fig. 1. Scheme for measuring the resolving power of the method of monochromatizing a beam of $\gamma$ quanta.

In this connection, work $^{1}$ is of interest, in which an attempt was made to “monochromatize” a beam of $\gamma$ quanta from a synchrotron with a maximum electron energy $E_0$ equal to 310 MeV. The basic idea consists in isolating $\gamma$ quanta of a definite energy by using a coincidence scheme*). The electron, having been decelerated in the target, emerges

from it, possessing energy \(E_1\), with good accuracy equal to its initial energy \(E_0\) minus the energy of the \(\gamma\)-quantum emitted as a result of braking \((E_\gamma)\). If a detector is placed in the path of electrons possessing some specified energy, and at the same time another detector is used to register a \(\gamma\)-quantum or the product of the nuclear reaction caused by it, then coincidence pulses will correspond to the registration of \(\gamma\)-quanta in a definite energy interval. The practical implementation of the present proposal proved to involve a number of difficulties; in particular, it required the use of complex radio apparatus. (In the scheme used in the work, there were more than 300 tubes.) For technical reasons, placing the electron detector inside the accelerator chamber proved difficult. Therefore the scintillation crystal registering the electrons was installed outside the chamber near its wall (Fig. 1). The electrons were analyzed in the magnetic field of the accelerator. The energy of the detected electrons was chosen to be \(100\) MeV, in order to avoid appreciable scattering of them while passing through the glass wall of the chamber. Thus, for \(E_0 = 310\) MeV the \(\gamma\)-quanta had to have an energy of about \(210\) MeV. The crystal of the electron detector was mounted on a lucite light guide, \(5.8\) cm in diameter and about \(90\) cm long, connected to a photomultiplier of type 5819, protected from the action of the magnetic field of the accelerator pole by a special screen.

Fig. 2. Results of measuring the resolving power of the method for monochromatizing a beam of \(\gamma\)-quanta. The upper curve was obtained with a crystal \(2\) cm thick, the lower with a crystal \(4\) cm thick.

Verification of the monochromatization scheme was carried out with the aid of a magnetic pair spectrometer\(^2\), whose radiator—copper foil \(25\) microns thick—was placed directly in the beam of \(\gamma\)-quanta. To reduce the number of random coincidences, the beam was stretched in time to \(0.3\) μsec by changing the form of the envelope of the high-frequency voltage. Nevertheless, this small resolving power of the electronics (about \(1\) μsec) did not permit operation at high intensities. At a beam intensity corresponding to \(10^6\) “effective” quanta per minute (the number of “effective” quanta is equal to the energy flux,

*) It is natural, therefore, that such a method is inapplicable in work with nuclear emulsions, in measurements of induced activity, etc.

assigned to the maximum energy of the $\gamma$ rays in the spectrum), the maximum coincidence counting rate was several pulses per minute. In determining the resolving power of the monochromatization arrangement, the ratio of triple coincidences (electron detector, positron and electron counters) to double coincidences (electron and positron counters) was measured as a function of the spectrometer magnetic field, proportional to the energy of the $\gamma$ quanta (Fig. 2). At a mean energy $E_\gamma = 190$ MeV the spectrum has a half-width of $\pm 30$ MeV. (This also includes the resolving power of the spectrometer, amounting to about 10%.) Considering the question of the resolving power of a coincidence arrangement necessary for work with high-intensity beams, the authors come to the conclusion that the effect-to-background ratio is proportional to

$$ \frac{1}{2KT\ln E_0/E_1}, $$

where $T$ is the resolving time of the coincidence circuit, $K$ is the beam intensity in number of effective quanta per second, $E_0$ is the energy of the accelerated electrons, and $E_1$ is the energy of the detected electrons. It is stated in the paper that coincidence circuits encountered in the literature, in which the tube grid is locked by a negative signal, are unsuitable in this case because of the presence of a large number of low-amplitude background pulses. In the circuit used by the authors, employing 6V No. 6 tubes, the resolving time was $(4—5)\cdot 10^{-9}$ sec. In the measurements two identical coincidence circuits were used, one of which contained an artificial delay and served for the registration of accidental coincidences.

The described system for monochromatizing a beam of $\gamma$ quanta was used to investigate the angular and energy distribution of photoprotons from carbon (Fig. 3). The beam of $\gamma$ quanta from the synchrotron target

Fig. 3. Diagram for measuring the distributions of photoprotons from carbon: 1 — synchrotron target; 2 — internal electron detector; 3 — collimator; 4 — monitor target; 5 — telescope target; 6, 7, 8 — counters of the photoproton telescope; 9 — copper absorbers; 10, 11 — monitors.

was collimated in a lead block and, passing through the graphite target of the monitor (one of the monitors was a photoproton counter), struck the main target of the proton telescope. The latter consisted of three stilbene scintillation counters, with coincidences of pulses in the first two counters and anticoincidences in the third counter being registered. It is indicated that the coincidences were registered by a “slow” circuit, which ensured more stable and accurate operation. In this case it was necessary to limit the magnitude of the intensity of the $\gamma$-quantum beam and, consequently, also the counting rate of protons. Protons of different energies were registered by changing the thickness of the copper absorbers placed in front of the counters. For absolute calibration of the intensity

Fig. 4. Energy distribution of photoprotons from carbon at \(\theta = 60 \pm 15^\circ\). \(A\)—from the bremsstrahlung spectrum; \(B\)—from the monoenergetic spectrum of \(\gamma\)-quanta.

Fig. 5. Angular distribution of photoprotons with energy \(70 \pm 17\) MeV from carbon.

of a monoenergetic beam of $\gamma$ quanta, the telescope was placed directly in the beam and recorded pairs formed in a lead plate placed between counters 1 and 2. The results of a study of the energy distribution of photoprotons from carbon, produced by the bremsstrahlung spectrum and by a monoenergetic beam of $\gamma$ quanta, are presented in Fig. 4 for an angle $\theta = 60^\circ \pm 15^\circ$. As is seen from a comparison of the results, in the latter case a noticeably more distinct bend in the spectrum is observed at a proton energy equal to $\tfrac{1}{2}$ of the mean energy of the $\gamma$ quanta, i.e., 100–110 MeV, which is a convincing argument in favor of the “quasideuteron” model. The paper gives the result of a calculation of the energy distribution of protons under the assumption of this model and of the Fermi distribution of the momenta of particles in the nucleus. The effect of the elastic scattering of protons in the nucleus was also taken into account. The proton energy spectrum thus obtained agrees well with experiment. The angular distribution of photoprotons with $\varepsilon_p = 70 \pm 17$ MeV from carbon (Fig. 5) from a monoenergetic beam of $\gamma$ quanta proved to be somewhat more isotropic than the distribution caused by bremsstrahlung. The results obtained using the described method of monochromatization of a beam of $\gamma$ quanta agree with the data yielded by the usually used “difference” method[^4].

B. R.

REFERENCES

1. J. W. Weil and B. D. McDaniel, Phys. Rev. 92, 391 (1953).
2. De-Wire, Ashkin and Beach, Phys. Rev. 83, 505 (1951).
3. J. Fischer and S. Marshall, Rev. Sc. Instr. 23, 417 (1952).
4. J. C. Keck, Phys. Rev. 85, 410 (1952); 86, 629 (1952).