Production of polarized ?-quanta by scattering light photons off electrons accelerated in the Sirius synchrotron
Abstract Generated abstract
The paper describes a proposed and partially prepared method for producing polarized, quasi-monoenergetic gamma quanta at the Sirius synchrotron by Compton scattering ruby-laser photons from relativistic electrons. It estimates attainable photon energies, energy selection by collimation, expected yield, and background from residual-gas bremsstrahlung for electron energies up to 1.5 GeV. For the stated Sirius parameters, the authors calculate maximum gamma energies up to about 61 MeV, roughly 12,000 Compton photons per accelerator cycle, and a lower bremsstrahlung background under the chosen collimation. The article also outlines the experimental arrangement, beam alignment procedure, and timing system, concluding that a polarized 60 MeV gamma beam of experimentally useful intensity should be achievable.
Full Text
PRODUCTION OF POLARIZED $\gamma$-QUANTA BY SCATTERING LIGHT PHOTONS ON ELECTRONS ACCELERATED IN THE “SIRIUS” SYNCHROTRON
A. V. PESHKOV, B. N. KALININ, E. I. ALEKSANDROV, A. V. KARAKUSHEV
(Submitted by the seminar of the HE sector of the Nuclear Physics Research Institute)
Photon beams that are used for research in nuclear physics and elementary-particle physics are characterized by the following parameters: 1) energy $\omega$, 2) energy width $\dfrac{\Delta \omega}{\omega}\%$, 3) intensity $N$, 4) degree of polarization $P\%$, and 5) smoothness of the energy variation.
At present experimenters have at their disposal photon sources with energies up to $20\ \text{GeV}$; however, the photons then have a continuous spectrum and are unpolarized. Methods for obtaining monoenergetic and polarized photons have begun to be developed and applied only recently.
One of these methods for obtaining polarized and quasi-monoenergetic $\gamma$-quanta is Compton scattering of photons on relativistic electrons [1, 2]. In such an interaction, the maximum energy corresponding to photons scattered in the direction of motion of the primary electrons in a head-on collision of the electron and photon beams is given by the following expression:
\[ \frac{\omega_{\max}}{\varepsilon} = \frac{1}{1+\dfrac{\varepsilon_0^2}{4\varepsilon\omega_1}}, \tag{1} \]
where
$\omega_1$ is the energy of the incident photon,
$\varepsilon_0$ is the rest energy of the electron,
$\varepsilon$ is the total energy of the electrons.
(Here a system of units is used in which $\hbar=c=1$.)
It follows from this that $\omega_{\max} \gg \omega_1$, and increasing $\varepsilon$ and $\omega_1$ leads to an increase in the energy of the scattered $\gamma$-quanta, which, for $\varepsilon_0^2/4\varepsilon\omega_1 \ll 1$, tends to the energy of the electrons.
At the “Sirius” synchrotron, the maximum energy of photons obtained by Compton scattering of photons from a ruby laser ($\omega_1 = 1.78\ \text{eV}$) on electrons with energy $1\ \text{GeV}$ is $\omega_{\max}=27.8\ \text{MeV}$, and at the maximum electron energy $1.5\ \text{GeV}$, $\omega_{\max}=61.2\ \text{MeV}$.
It should be noted that, unlike bremsstrahlung, in the case under consideration the energy of the photons produced is uniquely related to the angle of their emission relative to the direction of motion of the primary electron, which makes it possible to select $\gamma$-quanta of a deter—
...of a certain energy. In order to select \(\gamma\)-quanta with
\[ \frac{\Delta \omega}{\omega}=10\%. \]
at an electron-beam energy of \(1\ \text{GeV}\), a collimator with an aperture diameter of \(5.2\ \text{mm}\) will be required at a distance of \(20\ \text{m}\) from the radiation source. Such a collimator cuts out an angle \(2.6\cdot 10^{-4}\ \text{rad}\). The power of lasers currently available makes it possible to obtain photon beams with an intensity sufficient for carrying out experiments using spark and bubble chambers. The number of photons formed within a specified angle in the scattering of laser photons by accelerated electrons in the accelerator chamber can be calculated from formula (3)
\[ W_{\gamma}=\frac{2W_e W_p L\,d\sigma\, n}{cAT}, \tag{2} \]
where
\[ \begin{aligned} W_e&\text{ — the number of electrons in the accelerator orbit,}\\ W_p&\text{ — the number of photons emitted by the laser during one pulse,}\\ L&\text{ — the length of the interaction region,}\\ d\sigma&\text{ — the differential cross section,} \end{aligned} \]
Fig. 1. Diagram of the arrangement of the experimental equipment. \(L_1\) — focusing lens, VK — vacuum chamber of the accelerator, \(M_1\) — electron-beam monitor, \(D_1\) and \(D_2\) — movable diaphragms, PP — rotating prism, OKG — ruby laser, \(M_2\) — laser-radiation monitor, K — collimator, S — spectrometer
\[ \begin{aligned} c&\text{ — the speed of light,}\\ A&\text{ — the area of the larger of the interacting beams,}\\ T&\text{ — the interaction time,}\\ n&\text{ — the number of revolutions of the electron beam during the interaction time.} \end{aligned} \]
For the Sirius synchrotron, \(W_e=1\cdot 10^{10}\ \text{el.}\), \(W_p=3.5\cdot 10^{19}\) at a laser power of \(10\ \text{J}\); \(L=157\ \text{cm}\), \(d\sigma=54\ \text{mbarn}\), \(A=0.15\ \text{cm}^2\), \(T=1.5\ \mu\text{s}\); the number of Compton photons is \(W_{\gamma}=12000\) per one operating cycle of the accelerator.
The polarization of the photons obtained by this method will be \(100\%\); moreover, the resulting \(\gamma\)-quanta may possess both circular and plane polarization.
The principal source of background in the registration of such photons will be bremsstrahlung in the residual gas. For the Sirius synchrotron, where the vacuum in the chamber is about \(2\cdot 10^{-6}\ \text{torr}\), and the length of the straight section is \(157\ \text{cm}\), the background will amount to approximately 750 bremsstrahlung photons during the interaction time at a collimation angle of \(2.6\cdot 10^{-4}\ \text{rad}\).
The layout of the experimental setup prepared by us for polarized \(\gamma\)-quanta is shown in Fig. 1. The laser radiation
(OKG), installed at an angle of \(90^\circ\) to the position of the equilibrium orbit on a straight section, is turned by a rectangular prism (PP) and directed through a focusing lens (\(L_1\)) into the vacuum chamber of the VK accelerator. A movable mirror (3) is used for aligning the lens (\(L_1\)). The scattered \(\gamma\)-quanta leave the tangential branch pipe, pass through the turning prism (PP), and enter the detector of the spectrometer (S).
Monitors (\(M_1\)) and (\(M_2\)) control the laser power and the number of accelerated electrons.
To determine the axis tangent to the electron orbit on the straight section, at the moment when the electron energy reaches the specified value, two movable diaphragms (\(D_1\)) and (\(D_2\)) are used, with photomultipliers located behind them. By moving the diaphragms, one can record the minimum of the synchrotron radiation, which corresponds to the position of the diaphragms on the sought axis. Diaphragms set at the radiation minimum define the direction of the laser beam. We photographed the synchrotron radiation from a matte screen installed on the tangential branch pipe with a high-speed cine camera. As a result it was established that the position of the electron beam on the straight interaction section is stable. Subsequently the diaphragms were set by means of photographic papers exposed to synchrotron radiation. The density distribution of the synchrotron radiation is such that it is possible to distinguish a minimum corresponding to the electron orbit with the smallest curvature. The presence of this minimum makes it possible to set the alignment diaphragms both vertically and horizontally, which cannot be done when the interaction takes place on a curved section of the electron orbit, as was done in a similar experiment on the 680 MeV synchrotron at FIAN [5].
Fig. 2. Block diagram of the device ensuring the meeting of the electron and laser beams at a definite moment of time.
To carry out the interaction at the moment when the energy of the accelerated electrons reaches the specified value, the circuit shown in Fig. 2 is used.
A pulse from the synchrogenerator (SG) of the “Sirius” synchrotron is fed to the laser ignition circuit (SP) and to the delay circuit (SZ). The delay circuit (SZ) delays the triggering of the time-selection circuit (VS) by 60 \(\mu\)sec; the latter starts the spectrometer (S) only during the laser emission. The laser is powered from the pump circuit (SN). The experimental technique is analogous to the technique used in [5]. At present the alignment of the laser has been completed, and work is under way to obtain the expected effect.
The work described makes it possible to hope that in the near future a polarized beam of \(\gamma\)-quanta with an energy of 60 MeV and an intensity sufficient for experiment will be obtained.
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