Observation of Deflection of a Beam of Multi-GeV Electrons by a Thin Crystal

We report on an experiment performing channeling and volume reflection of a high-energy electron beam using a quasimosaic, bent silicon (111) crystal at the End Station A Test Beam at SLAC. The experiment uses beams of 3.35 and 6.3 GeV. In the channeling orientation, deflections of the beam of $400\mu \mathrm{rad}$ for both energies with about 22% efficiency are observed, while in the volume-reflection orientation, deflection of the beam by $120\mu \mathrm{rad}$ at 3.35 GeV and by $80\mu \mathrm{rad}$ at 6.3 GeV is observed with 86%–95% efficiency. Quantitative measurements of the channeling efficiency, surface transmission, and dechanneling length are taken. These are the first quantitative measurements of channeling and volume reflection using a primary beam of multi-GeV electrons.

Figure 1

Top view layout of the experiment.

Figure 2

Deflection plot at 6.3 GeV beam energy. Colors correspond to log(intensity). The numbers denote the orientation of the crystal. 1: amorphous. 2: channeling. 3: dechanneling. 4: volume reflection. 5: volume capture.

Figure 3

Example of the fit to 6.3 GeV channeling data at 0 mrad crystal angle. The dashed line (green) is the fitted function; the crosses are the measured intensity. The solid dark-gray lines represent the individual contributions to the fit (Gaussian peaks, dechanneling tail, and constant background). For this example, the channeling efficiency is 23.8%, the surface transmission is 55%, and the dechanneling length is $33\mu \mathrm{m}$. The maroon histogram is from DYNECHARM++ simulations, see text.

Figure 4

Example of the fit to 3.35 GeV channeling data at 0 mrad crystal angle. Plot symbols and lines have the same meaning as in Fig. 3. For this example, the channeling efficiency is 22%, the surface transmission is 62%, and the dechanneling length is $38\mu \mathrm{m}$.

Figure 5

Dechanneling length versus crystal angle, 6.3 GeV. Error bars reflect the scattering of the underlying data from several frames; the data points at $-0.05$ and $+0.045\mathrm{mrad}$ have larger uncertainty due to beam jitter.

Figure 6

Published data for the dechanneling length of electrons in Si crystals. The two open boxes indicate data for (110) planar channeling in straight crystals [33]; the open circles, (110) straight crystal [7]; the open diamonds, (111) straight crystal ([8] Table 1); the solid circles are this work, (111) bent crystal.

Figure 7

Vertical profiles of channeled (black $+$) and not-channeled (brown $\times$) beam at 6.3 GeV together with Gaussian fits (dashed lines). The $1\sigma$ width of the channeled particles is $60\mu \mathrm{rad}$, while the $1\sigma$ width of the not-channeled particles is $35\mu \mathrm{rad}$, consistent with $39\mu \mathrm{rad}$ from the multiple-scattering formula [38]. The data sets are intensity normalized to each other.