Radiation from 10 GeV positrons channeled in silicon crystals

Nuclear Instruments and Methods 194 (1982) 239-241 North-Holland Publishing Company

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RADIATION FROM 10 GeV POSITRONS CHANNELED IN SILICON CRYSTALS N . A . F I L A T O V A , V.M. G O L O V A T Y U K , A . N . I S K A K O V , I.M. I V A N C H E N K O , R.B. K A D Y R O V , N . N . K A R P E N K O , T.S. N I G M A N O V , V.V. P A L C H I K , V.D. R I A B T S O V , M . D . S H A F R A N O V , E.N. T S Y G A N O V , I.A. T Y A P K I N , D.V. U R A L S K I Joint Institute for Nuclear Research, Dubna, U.S.S.R.

A. F O R Y C K I , Z. G U Z I K , J. W O J T K O W S K A Institute for Nuclear Research, Swierk, Poland

R.A. C A R R I G A N , Jr., T.E. T O O H I G Fermi National Accelerator Laborato~, Batavia, I1., U.S.A.

C. C A R M A C K , W . M . G I B S O N , I.-J. K I M , C.-R. S U N State University of New York at Albany, Albany, N. Y., U.S.A.

M . D . B A V I Z H E V , N . K . B U L G A K O V , N.I. Z I M I N Tomsk Polytechnical Institute, Tomsk, U.S.S.R.

I.A. G R I S H A E V , G . D . K O V A L E N K O ,

B.I. S H R A M E N K O

Kharkov Physical-Technical Institute, Kharkov, U.S.S.R.

E.I. D E N I S O V , V.I. G L E B O V Kurchatov Institute of Atomic Energy, Moscow, U.S.S.R.

and V.V. A V D E I C H I K O V V.G. Khlopin Radium Institute, Leningrad, U.S.S.R.

Radiation from 10 GeV positrons channeled along (I 10) planes in a 90 #m silicon crystal shows a prominent photon peak at about 50 MeV in agreement with theoretical predictions. The intensity is also in reasonable agreement with expectation. For positrons aligned within~20 grad to the planar direction, a sharp spectral peak of 25 MeV is also observed, the origin of which is not yet clear. For particles incident at angles near the channeling critical angle of~65 #rad there appears to be structure in the photon spectrum which may correspond to harmonic structure predicted by theory. This is the first observation of such structure for ultrarelativistic particles.

Radiation from channeled electrons and positrons has now been observed for particle energies from a few MeV to a few GeV [1-3]. In order to investigate this radiation further at high energies a collaborative U.S.S.R.-U.S. study is underway at the 76 GeV I.H.E.P. accelerator at Serpukhov in the U.S.S.R. This report will summarize some initial results from this study. In addition to the usual array of Cherenkov counters, drift chambers and scintillation detectors used in high energy channeling experiments for particle identifi0029-554X/82/0000-0000/$02.75 © 1982 North-Holland

cation and to define the incident and emergent particle trajectories, a cesium iodide spectrometer was introduced for photon indentification and measurement and a secondary particle spectrometer for electron/positron identification and energy measurement. Sources of background radiation were minimized by limiting the amount of radiating material in the beam and by introduction of weak magnets to separate upstream photon background sources. In this way photons produced in the apparatus other than in the crystal or in the drift V. CHANNELING RADIATION

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N.A. Filatova et al. / 10 Ge V posttrons chamwled m Si co'staL~

chamber module adjacent to the crystal, were not included in the aperture of the photon spectrometer. To further reduce the background, the target drift chamber was operated at low pressui'e and reduced in mass so that it represented a thickness of only 4 × 10-4 radiation lengths. Other than the drift chamber, the particle trajectory both upstream and downstream of the target was entirely in vacuum. Background measurements cartied out with no target crystal present showed less than one percent of the measured intensity in the spectral region of interest. Photons originating in the crystal and emitted in the beam direction in a cone o f - 1000 microradians apex angle were measured in a CsI(T1) photon spectrometer. The CsI(T1) was shielded by lead and surrounded by anticoincident detectors. The detector was a cylindrical crystal of 150 mm diameter with a length of 320 mm (approximately 13 radiation lengths). The spectrometer was calibrated with Cs, Co and Po-Be sources and monitored during the measurement by periodically adjusting the triggering condition to accept minimum ionizing muons. All events were recorded which produced a photon of energy greater than 5 MeV in the CsI(T1) detector and a positron signal in the lead glass Cherenkov counter array. To eliminate events due to the low energy tail of the incident positron beam, the minimum energy of the positron as determined by the downstream analysis magnetic spectrometer was required to be greater than 8 GeV. For each event, the incident direction relative to the (110) plane, the photon energy and the secondary positron energy were recorded. Fig. 1 shows the spectral density of the observed radiation as a function of photon energy for all events lying within 50/~rad of the (110) plane (the calculated critical angle is 65 p.rad). A line has been fitted through the points consistent with the apparatus resolution. This spectrum is in reasonable agreement with that predicted by the calculations of Kumakhov [4] which predicts that the maximum should be near 50 MeV and gives very nearly the same spectral shape. The intensity of the radiation is also in good agreement with Kumakhov's calculation. Dependence of the photon intensity on incident particle direction for particles with energy in the region of the peak at 50 MeV and far removed from the peak is shown in figs. 2a and b. The width of the maximum shown in fig. 2a for radiation between 30 and 80 MeV is in agreement with the expeccted critical channeling angle confirming that the peak of the radiation is associated with channeled particles. On the other hand the intensity of very high energy photons as shown in fig. 2b has a minimum in the planar direction with a maximum at angles several times the critical angle. It is expected that coherent bremsstrahlung will be a major source of radiation in this energy range and this angular

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dependence is consistent with that expectation. Positrons incident in directions very close to the planar direction produce a spectrum with an additional feature that has not been observed previously and is not predicted by the theory. This is a sharp peak a t - 25

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N.A. Filatova et a L /

10 GeV positrons channeled m Si crystals

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Fig. 3. a) Spectral density of radiation for positrons incident on the silicon crystal i n the angular range from zero to 20 #rad relative to the (I 10) planar direction. This is much less than the critical angle of 65/~rad. The solid line is a fit to the experimental data taking the system resolution and statistical fluctuation into account. The dashed line is a theoretical calculation [4}. The dotted line shows the results [3] for diamond scaled to correspond to 10 GeV positrons in silicon. Points are also shown for measurements of bremsstrahlung from a polycrystalline aluminium sample, b) Spectral density of photons produced by positrons incident on the crystal in the angular range 5 0 < 0 < 8 0 #rad relative to a (110) planar direction. This range brackets the critical angle of 65/~rad.

MeV as shown in fig. 3a. The origin of this narrow photon peak is not understood. It decreases rapidly with increased incident angle. In addition there appears to be structure at energies higher than the principle peak at ~ 50 MeV. Also shown on this figure are predictions from the Kumakhov theory and results from measure-

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ments in diamond [3] scaled to the appropriate energy for 10 GeV positrons in silicon. There appears to be good overall agreement between the two sets of measurements and with the theory except for the peak at low energy. Measurements with a polyerystalhne aluminum target is also shown on this figure. This shows the contribution from regular bremsstrahlung and demonstrates that it has an intensity much lower than the channeling radiation in the angle and energy range used in this study. As the angle of the incident positrons is increased, the indication of structure in the photon spectrum at energies above 50 M e ¥ persists. Fig. 3b shows the spectrum for incidence in the region of the critical angle. Even though the statistics are low there appear to be peaks in the spectrum. These are consistent with harmonic structure predicted by Kumakhov [4] for very high energy particles. Additional measurements are bing carried out to investigate the details of the photon spectrum, in particular the energy-angle relationships in order to better define the origin of the low energy peak and the apparent high energy structure. The authors are indebted to Professor N.N. Bogolubov, Professor A.M. Baldin, and Professor Leon Lederm a n for their continued encouragement and support. We are especially grateful to Professor A.A. Logunov and the staff of the IHEP accelerator for their help, especially in the design of the electron/positron beam channel.

References

[1] J.U. Andersen and E. Laegsgaard, Phys. Rev. Letters 44 1079 (1980); N. Cue, E. Bondemp, B.B. Marsh, H. Bakhru, R.E. Benenson, R. Haight, K. Inglis and G.D. Williams, Phys. Lett. 80A (1980) 26. [2] M.J. Alguard, R.L. Swent, R.N. Pantell, B.L. Berman, S.D. Bloom and S. Datz, Phys. Rev. Lett. 42 (1979) 1148, and 43 (1979) 1723. [3] I.I. Miroshnichenko, D.D. Murray, R.O. Avakyan and T.X. Fieguth, Pisma v JETP (USSR) 29 (1979) 786. [4] M.A. Kumakhov, Phys. Letters 57A (176) 17; Zh. Eksper. Teor. Fiz. (JETP,USSR) 72 (1977) 1489; See also R. Wedell, Phys. Stat. Sol. (b) 99 (1980) 11.

V. CHANNELING RADIATION