New Method for Producing and Analyzing Linearly Polarized Gamma-Ray Beams

Abstract

In this note we propose a new method for the production and the analysis of a polarized beam of high-energy gamma rays. The method is based on the interference effects which are observable in high-energy electron pair production on crystals. As a consequence of these the absorption rate (inverse of the mean free path) of very highenergy photons in crystal matter depends on their linear polarization. This suggests the possibility of using a thick crystal for the polarization and analysis of high-energy gamma rays. The polarization is effected by preferential absorption of the unwanted polarization component, and the analysis by transmission measurements, as in the case of a polaroid filter for visible light. Extensive theoretical work on the interference effects of high-energy electrodynamic processes in crystals has been done by Uberall,^ and refined experiments have given results which are in excellent agreement with the theory.^ Another method for the production of linearly polarized gamma rays by means of interference effects in a crystal is based on bremsstrahlung.^ In this case the theory is also in excellent agreement with the experimental results obtained with electrons of --1 GeV.^ As a polarimeter the device we propose is perhaps unique in the high-energy region, where the application of other methods based on the angular distribution in pair production and elastic photoproduction of 7r° on nuclei of zero spin^ requires very difficult experiments. A unique characteristic of the device is that, since it is both a polarizer and an analyzer, one can build two or more of them and cross-calibrate them with each other. The polarizing power of the device can therefore be directly measured.The absorption of high-energy photons is mainly due to electron pair production, a process which is already known to give interference effects.^'^ Let us consider the case of a cubic crystal, where the momentum k of the incoming gamma rays is in the (001) plane and makes a small angle a with the (110) axis. The (001) plane is then a symmetry plane. We find that the total cross section for pair production depends in this situation on the linear polarization of the gamma rays.Let us denote by z" and T>^ the total cross sections per unit volume of the crystal for gamma rays which are linearly polarized in the (001) plane and orthogonal to it. The two polarization components will be absorbed with different mean free paths; i.e., after having penetrated a thickness X of the crystal the intensities of the two components will be reduced according to® /V)=/"(0)exp[-T:V],If the beam was originally unpolarized [/" (0) = / (0)], we now have a polarization: P{x)-[l\x) -l''{x)]/[l\x)^I^{x)]^tanh[iAr(S -S )].(2) From Eq.(2) one can see that this method could, in principle, produce any degree of polarization, with an appropriate choice of the thickness x. This is achieved with a loss of the original intensity which can be expressed in terms of the polarization P{x) and of a param-270

Cosmic system dynamics: a cyberneticist's perspective on gravitation

“…(1) Photons and elementary particles have been shown to be interchangeable (Landau and Lifshits, 1934;Cabibbo and Gatto, 1961;Cabibbo et al, 1962;Di Vecchia and Greco, 1967;Barbiellini et al, 1974;Schwitters and Strauch, 1976;Baldini et al, 1979;Cooper, 1988;Bernardini, 2004;Ginzburg, 2006). (2) The phenomenon of zitterbewegung, variation at the speed of light of an electron's position about its mean path as first identified by Schrödinger (1930), has been shown to be a demonstrable physical feature of the composition of that particle (Catillon et al, 2008).…”

Coherent bremsstrahlung, coherent pair production, birefringence, and polarimetry in the 20–170 GeV energy range using aligned crystals

“…In this case, the photon polarization vector e will be the combination of two unit vectors, t and y, parallel and perpendicular to the reaction plane, respectively: e e k t e ? y: (11) The components of the polarization vector before and after the crystal of thickness, L, are related by a 2 2 matrix [51,52]:…”

“…are complex quantities analogous to the index of refraction. The imaginary part of the index of refraction is connected with the photon absorption cross section, while the real part can be derived from the imaginary part using dispersion relations [51]. The crystal can act as a quarter wave plate, if the real part of the relative phases of the two components of the waves parallel and perpendicular to the reaction plane is changed by =2 after transmission of the photon.…”

The trigger and DAQ systems of the Na59 experiment

“…One way of realizing such a beam is to start with polarized electrons and obtain high energy photons through laser backscattering [1]. Another method, albeit untested until recently, is that proposed by Cabibbo [2], which does not require initial beam polarization. Instead, it makes use of the coherent effects in single crystals.…”