Experimental study of the radiation emitted by $180\text{−}\mathrm{GeV}∕c$ electrons and positrons volume-reflected in a bent crystal

The radiation emitted by $180-\mathrm{GeV}∕c$ volume-reflected electrons and positrons impinging on a bent crystal has been measured by the H8RD22 Collaboration on the H8 beamline at the CERN SPS. A dedicated spectrometer has been developed to measure high-energy photon spectra (up to $\sim 100\mathrm{GeV}$) under volume reflection: photon and charged particle beams have been separated by a bending magnet and leptons were detected and tagged by microstrip silicon detectors and a Pb-scintillator sampling calorimeter. A comparison between the experimental and analytical data for the amorphous and volume-reflection cases is presented and the differences are discussed.

Figure 1

(Color online) Volume-reflection phenomenon. (a) A charged particle in the crystal volume reflected at the turning radial coordinate $r_t$. (b) The particle transverse energy as a function of the radial coordinate.

Figure 2

Relative transverse velocities $(v_t∕c)$ of $180\text{−}\mathrm{GeV}∕c$ positrons (a) and electrons (b) at volume reflection in a (111) silicon plane as a function of time (in femtoseconds). The crystal thickness is $0.84\mathrm{mm}$, and the bending radius is equal to $12\mathrm{m}$ for positron and $8\mathrm{m}$ for electron beams, according to the experimental conditions.

Figure 3

(Color online) Maximum energies of the $\gamma$ quanta (a) calculated according to Eqs. (1, 2) from the corresponding frequencies of the motion. Red (top) and green (bottom) lines correspond to $180\text{−}\mathrm{GeV}∕c$ positrons and electrons, respectively. The shapes (b) of the $\gamma$-quantum spectra and energy losses in periodic structures: the gray $\left(A\right)$ and green $\left(B\right)$ curves are the $\gamma$ spectrum and distribution of energy losses for a structure with one period. The blue $\left(C\right)$ and red $\left(D\right)$ curves are the same values but for an infinite periodic structure.

Figure 4

(Color online) The setup: the microstrip silicon telescope $\left(\mathrm{Si}X\right)$, the high-precision goniometer (g), a bending magnet (BM), the $9.5\times 9.5{\mathrm{cm}}^2$ silicon beam chambers (BC3, BC4), a pair of scintillators $\left(\mathrm{Sci}X\right)$, and the electromagnetic calorimeter (DEVA).

Figure 5

(Color online) The DEVA calorimeter in the H8 beamline: DEVA is formed by 12 plastic scintillator tiles and 11 lead tiles for a total of $13.07X_0$. The light produced in the scintillators is brought out by WLS fibers to a 16-channel PMT.

Figure 6

The H8 beam spectrum measured by the DEVA calorimeter: the first peak corresponds to muons, the central region is populated by hadrons, while the last peak consists of light leptons.

Figure 7

(Color online) Quasimosaic crystals on their holder. QM2 is the one on the left, QM1 on the right.

Figure 8

(Color online) Horizontal incoming angle distribution.

Figure 9

(Color online) (a) Angular scan: the $x$ axis is the goniometer angle, that is, the crystal angle with respect to the beam, and the $y$ axis is the deflection angle defined as the difference between the incoming and the outgoing angles. (b) The deflection angle distributions in the amorphous (gray) and VR (white) positions: the particles in volume reflection are steered by about $25\mu \mathrm{rad}$ with a high efficiency. The VR Gaussian left tail (which corresponds to the particles that have lost energy) is much higher than the amorphous one.

Figure 10

(Color online) The spectrometer method: a particle in VR can emit photons (yellow dashed line), decreasing its momentum. A bending magnet steers particles depending on their momentum: the solid red line corresponds to a $180\text{−}\mathrm{GeV}∕c$ particle, while dashed black and dotted blue lines correspond to particles that have lost energy. Silicon detectors (light blue) are able to reconstruct the particle trajectories.

Figure 11

Energy loss distribution for the volume-reflection case.

Figure 12

(Color online) Amorphous spectrum for an electron beam: the figures show a good agreement between experimental data (blue dots) and calculated (black line) and simulated (red line) ones.

Figure 13

(Color online) Volume-reflection spectrum for positrons (a) and electrons (b): experimental data (red and green dots) are compared with the calculated ones (black line).

Figure 14

(Color online) Energy loss spectra [$(dN∕dE)E$ vs $E$] for the positron (a) and electron (b) cases: the blue dots are the experimental amorphous data, the red and green dots represent the volume-reflection experimental results, the black lines identify the volume reflection and amorphous calculated spectra.