Radiative moderation of an electron beam by a silicon crystal

Nuclear Instruments and Methods 216 (1983) 93-97 North-Holland Publishing Company

RADIATIVE

MODERATION

A.M. FROLOV,

OF AN ELECTRON

K.I. G U B R I E N K O ,

93

BEAM

V.A. M A I S H E E V ,

BY A SILICON

A.I. M Y S N I K ,

CRYSTAL

S.B. N U R U S H E V

a n d V.L. S O L O V I A N O V Institute for High Energy Physics, Serpukhov, Moscow District, USSR Received 3 January 1983

Intensity decrease of a 26.6 G e V / c electron beam with angular divergence of about _+ 1 mrad caused by radiative energy losses in an oriented silicon crystal 0.34 r.l. thick was measured. Beams of moderated electrons in the momentum region of 7.5 17.5 G e V / e were obtained. Strong orientation effects were observed for intensities of all beams. The experimental results are satisfactorily described with the known relations for coherent bremsstrahlung in crystals.

1. Introduction Electron (positron) beams with m o d e r n p r o t o n accelerators allow one to study electromagnetic processes in m a t t e r at energies up to tens and h u n d r e d s of G e V not a t t a i n a b l e at operating electron machines. G r e a t attention was therefore recently paid to the theoretical a n d experimental p r o b l e m s of electromagnetic interactions in orderly crystalline structure. Interesting experimental results on coherent b r e m s s t r a h l u n g of electrons in crystals, p r o d u c t i o n of electromagnetic showers in them, s p o n t a n e o u s radiation of electrons a n d positrons channeled in the crystal were o b t a i n e d in the new energy region. T h e present p a p e r gives a description of an experim e n t carried out at the Serpukhov 70 G e V p r o t o n accelerator where radiative m o d e r a t i o n of a highm o m e n t u m electron b e a m by an oriented silicon crystal was investigated. Practical application of this effect to o b t a i n electrons with decreased m o m e n t a is considered.

electron b e a m which was then formed by further optics M 2 - Q 6. High m o m e n t u m electrons were radiatively moderated by a silicon crystal C R in a goniometer. The goniometer with the crystal was placed in the convergent b e a m between the head quadrupoles and analyzing m a g n e t within the v a c u u m line of the transport system. Lower m o m e n t u m electron beams were formed b e h i n d the crystal when fields a n d gradients in magnetic elem e n t s M 1 - Q 6 decreased proportionally to m o m e n t u m . O n e could also have shower positrons from the crystal in the transport system if supply polarities for these magnetic elements were inverted. The m e t h o d described has been used earlier to decrease electron m o m e n t u m in b e a m 14E with the help of a n a m o r p h o u s radiator [2]. The electron b e a m could be stopped in the t r a n s p o r t system by a copper filter, BS 1, 77X 0 thick b e h i n d the m a g n e t M l or with the help of lead filter, BS 2, 4X 0

2. E x p e r i m e n t a l layout The experiment was carried out in the high-energy electron b e a m 14E [1]. The a p p a r a t u s of the operating physical setup P R O Z A [2] was used. The two-internaltargets procedure for b e a m generation (p + N ---, 7r° + X, ~ r ° ~ 2 y in the h a d r o n i c target, a n d y---, e - e + in the converter) is described in ref. 3. The b e a m forming in the magnetic t r a n s p o r t system is illustrated in fig. 1. The electron b e a m extracted from the accelerator is limited by the aperture collimators C I - C 2 a n d then is c a p t u r e d by the h e a d q u a d r u p o l e objective Q I - Q 2 in the transport system. The analyzing magnet M 1 a n d m o m e n t u m slit C 3 b o t h selected a q u a s i - m o n o c h r o m a t i c 0 1 6 7 - 5 0 8 7 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d

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94

A.M. Frolot; et aL / Radiative moderation of an electron beam

thick immediately before the collimator C 3 (where X 0 is a radiation length). Beam i n t e n s i t u in the transport system was monitored with external scintillation counters SI~,-S2~,. These counters measured the m u o n flux accompanying proton interactions in the h a d r o n i c target. The beam itself was m o n i t o r e d by thin scintillation counters S~ S 2 and could be redefined by counter S 3 placed in the focal plane of the transport system. The first two counters were e 80 × 5 m m 2 while the last one could be varied in sizes. Beam profiles before the counter S 3 were controlled with one-coordinate scintillation hodoscopes G~ G 2 in the horizontal and vertical planes. Particle energy was measured in the formed beam by the leadglass C h e r e n k o v hodoscope ( ' (further TACH). The energy resolution achieved with T A C H in the 40 G e V electron beam was about 5% (the width of amplitude distribution at half-height). The a m o r p h o u s m a t t e r with summed thickness of 0.2 X 0 available in the p a t h Q6 ~ spread the energy distribution of electrons in the formed b e a m to lower energies. This did not however influence the measured particle intensity in the formed beam.

3. Crystal and goniometer In the experiment we used as a radiator a silicon crystal plate of 75 m m diameter with thickness of 0.34 X o (in silicon X 0 = 9 cm). The plate was cut out from a single crystal along the (100) planes. This radiator was fixed in the remote controlled frame of the goniometer a n d could be put i n / o u t of the electron beam. The vertical and horizontal goniometric axes were adjusted in the electron b e a m cross section. The goniometer frame could not rotate around the vertical axis, i.e. corresponding orientation angle of the crystal was q)v = const. It was possible to orient the crystal only about the horizontal goniometric axis within a range q)H -- --+7° with a rotation angle accuracy of + 0.2 mrad. Denote by b~, b 2, b 3 the basis vectors aligned with the corresponding crystal axes [100], [011] and [0II]. The v e c t o r s b 2 and b 3 were adjusted mechanically along the vertical and horizontal goniometric axes, respectively. The angles were assumed to be q)v = ~ n = 0 if the vector b I was aligned with the b e a m axis. According to experimental results the radiator was rotated at the angle q~v = 7 mrad while the v e c t o r b 2 was turned relative to the vertical direction by an angle of 53 mrad in the c o m m o n plane.

4. Electron beams The transport system was first adjusted for primary electrons with central m o m e n t u m P0 = 26.6 G e V / c and

s t a n d a r d deviation op/po = _+ l%. The electron beam was experimentaly limited by the aperture collimators to just reject particles that bypass the radiator when it is set up in working position. The calculated angular divergences of the primary electrons in the crystal can be described by G a u s s i a n distributions with s t a n d a r d deviations - _+(0.60 × 0.75) mrad 2 in horizontal and vertical planes, respectively. Measured beam intensity was 3 × 10 4 electrons per 5 × 10 n accelerated protons in 1 s pulse. The c o n t a m i n a t i o n of ~r and p~ mesons was less than 0.5% in the electron beam. The focal spot was 25 m m in diameter. Secondary electron beams (primary electrons moderated by the crystal) were formed to calibrate the experimental facilities. Here the m o m e n t u m slit remained the same as for primary electrons. These electron beams had a central m o m e n t u m varied within the interval p 6 - 17.5 7.5 G e V / c , m o m e n t u m deviation ol,,/p(~ ~ + 8%, focal spot attained of - 50 × 70 ram2 in horizontal a n d vertical planes, respectively. The two last were caused by the disarray of the transport system optic that is due to multiple scattering and energy loss spread of electrons in the radiator.

5. Experimental results and discussion Primary a n d secondary electron b e a m intensities were investigated in the transport system when the orientation angle of crystalline radiator q)H was varied. The primary electrons h a d m o m e n t a p -- 26.6 + 0.3 G e V / c . The main measurements with secondary electrons were m a d e for the m o m e n t a p ' - - 1 0 + 0.8 G e V / c . Experimental results are presented in fig. 2. These results show a selective change of the beam intensities versus the variable angle. The orientation dependences in fig. 2 are explained by the initial a d j u s t m e n t of the crystalline radiator in the primary electron beam. The experimental situation is illustrated in fig. 3 where the so-called map of the basic straight lines [4] is shown for the used radiator. The straight line F F ' on the map describes an angular rotation of the radiator in the experiment (q~u = var, q~v = 7 mrad). W h e n the angle q), corresponds to cross points of the line F F ' with the basic lines then peaks of a coherent b r e m s s t r a h l u n g radiation by the primary electrons should be observed and consequently dips or peaks of intensity in figs. 2(a) and (b), respectively. Each dip or peak has a width depending on the initial a d j u s t m e n t of the radiator, vertical and horizontal angular divergences of the primary electrons and the multiple scattering of particles in the radiator. The beams were calculated by the M o n t e Carlo method. The b r e m s s t r a h l u n g radiation in a single crystal [5,6] and the multiple scattering were taken into account in the calculations. The atomic form factor for silicon

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F Fig. 3. Map of general basic straight lines for coherent bremsstrahlung radiation by electrons that have passed the silicon single crystal in the experiment; ~H, q~v - orientation angles of the crystal around horizontal and vertical goniometric axes; n z, n 3 - integer indices of points in the reciprocal lattice for silicon crystal in directions b2=[011 ] and b3=[011]; the straight line FF' shows how the angle orientation of the vector b I = [100] changed with respect to the beam axis in the experiment.

N ~ was taken f r o m ref. 7 (the Moli6re potential). A t c o n s i d e r a b l y large a n g u l a r s p r e a d o f the b e a m in n o t too thin a crystal, the effect o f s p o n t a n e o u s r a d i a t i o n f r o m the c h a n n e l e d e l e c t r o n s was negligible. S h o w e r p r o d u c t i o n in the r a d i a t o r was n o t c o n s i d e r e d . T h e calculations were m a d e with _+ 3% accuracy. It was verified that t h e r e is n o t a s t r o n g d e c r e a s e of high energy particles flux in the t r a n s p o r t s y s t e m d u e to multiple scattering in the crystalline r a d i a t o r in the 40 G e V ~r--meson b e a m at the set-up P R O Z A a n d the d e c r e a s e t u r n e d out to be a b o u t 6%. N o o r i e n t a t i o n effects were o b s e r v e d in these m e a s u r e m e n t s . T h e flux o f s h o w e r p o s i t r o n s was N e , / N e - 3 % at m o m e n t a p ' = 10 _+ 0.8 G e V / c in the t r a n s p o r t system. C a l c u l a t e d o r i e n t a t i o n curves are p r e s e n t e d in fig. 2 for the e x p e r i m e n t a l c o n d i t i o n s . T h e curves are n o r m a l ized to the m e a s u r e d flux o f p r i m a r y e l e c t r o n s in the t r a n s p o r t s y s t e m a n d they are in s a t i s f a c t o r y a g r e e m e n t

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96

A.M. Frolov et al. / Radiative moderation of an electron beam

tors of different thicknesses for the experimental conditions. It can be seen that one may expect an increase of secondary electron fluxes in the transport system in comparison with the ones measured if the crystalline radiator has the optimal thicknesses of 0.15-0.65 X 0 in the above-mentioned region of p(). The fluxes must be increased 1.1-1.5 and 2 times for the hard and soft parts of the momentum region p('), respectively. The background problem was investigated by the difference method for beam intensities, when the converter in the accelerator was out of the working position or the filters in the transport system were placed in the beam. Also direct measurements were made here with the help of T A C H . Background fluxes of N b / N +, - 0.7-1 were observed in the beam intensity maxima ( ~ H = --131 mrad and + u = + 2 mrad) at momenta p ~ 7.5-15 G e V / c , respectively. This background consisted mainly of rr+ and ~ mesons from the hadronic target placed in the accelerator. When beams were redefined by the scintillation counter with diameter of 20 mm in the focal spot (S 3 in fig. 1) their intensities decreased by nearly order of magnitude. The momentum spread of the electrons decreased 2-2.5 times and the background ratios became N b / N ~, - 0.2 0.3 in the redefined beams. Considerable electromagnetic background of N b / N +, ~ 3 was observed in the unredefined beams at momenta p~ > 17.5 G e V / c . This occurs because of the positions of the magnetic elements in the transport system and its simple optics. Characteristic energy spectra of particles in the beams obtained with the help of T A C H are illustrated in fig. 7. .A single crystal 0.34 X 0 thick in an electron beam with m o m e n t u m P 0 - 26.6 G e V / c gave an increase in intensities of moderated electrons with momenta p~

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with experiment. Note that the crystalline radiator used had almost optimum thickness for production of an intense secondary electron beam with p~ = 10 G e V / c in the experiment (fig. 4). Measured and calculated fluxes of secondary electrons in the transport system with central momenta within the range p ~ - 17.5-7.5 G e V / c are presented in fig. 5. Fig. 6 shows calculated momentum spectra for secondary electrons immediately after crystalline radia-

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A.M. Frolov et al. / Radiative moderation of an electron beam

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7 - 1 5 G e V / c respectively 1.5-2 times in comparison with an a m o r p h o u s radiator of optimal thickness 0 . 6 X 0. There were also lower b a c k g r o u n d levels and u n w a n t e d effects due to multiple scattering of particles in a radiator. High energy electron b e a m s 14E [1] a n d 2E [3] are produced at the Serpukhov accelerator with the help of one a n d the same target station in two magnetic transport systems. These t r a n s p o r t systems have a c o m m o n h e a d path where the goniometer with crystal was placed. This allowed us to verify the general experimental results in the more qualitative electron b e a m 2E. Also m o d e r a t e d electron b e a m s were o b t a i n e d here for calibration of particle detectors [8].

c o m p a r i s o n with an a m o r p h o u s radiator. N o t thick orie n t e d c r y s t a l s c a n b e u s e d for r e j e c t i o n of e l e c t r o n - p o s i t r o n directional background. It is interesting also to use crystalline plates for shower p r o d u c t i o n in t o t a l - a b s o r p t i o n detectors with orientational specialities. The authors t h a n k V.I. Kotov for attention to the work and they are grateful to V.A. Pichugin, L.N. Korolev, A.P. Meshchanin, Yu. A. Matulenko, A.N. Vasiliev, L.F. Soloviev, V.D. Apokin, B.V. Chuiko for their helpfulness.

References 6. Conclusion In the experiment considerable probability increase was observed for radiative m o d e r a t i o n of quasi-monoc h r o m a t i c electrons with m o m e n t a near 26.6 G e V / c a n d a noticeable divergence of a b o u t _+ 1 mrad in not thick 0.34 X 0 oriented silicon single crystal. The effect d e p e n d e d strongly o n angular o r i e n t a t i o n of the crystal. Experimental results for b e a m intensity a t t e n u a t i o n a n d p r o d u c t i o n of electrons with decreased m o m e n t a are satisfactorily described with the help of the k n o w n relations for b r e m s s t r a h l u n g in single crystals. N o particularities were observed for multiple scattering of high energy particles in the crystal. The experiment showed that a crystal can be adj u s t e d in the electron b e a m using here the orientation effects in intensity a t t e n u a t i o n or p r o d u c t i o n of electrons with decreased m o m e n t a . The use of a crystal gave a noticeable increase in yield of m o d e r a t e d electrons in

[1] V.A. Maisheev, A.M. Frolov, E.A. Arakelyan, G.L. Bayatyan, G.S. Vartanyan, N.K. Grigoryan, A.T. Margaryan and S.S. Stepanyan, Preprint IHEP 76-15 (Serpukhov, 1976). [2] I.A. Avvakumov et al., Preprint IHEP 81-15 (Serpukhov, 1981). [3] S.S. Gershtein et al., Nucl. Instr. and Meth. 112 (1973) 477; Atomnaya Energia 35 (1973) 181; Preprint IHEP 72-93 (Serpukhov, 1972). [4] A.M. Frolov et al., Nucl. Instr. and Meth., 178 (1980) 319; JETP 77 (1979) 1708; Sov. Phys. JETP 50(5) (1979) 856 (Edit. Amer. Inst. of Physics); Preprint IHEP 79-63 (Serpukhov, 1979). [5] G. Diambrini, Rev. Mod. Phys. 40 (1968) 611. [6] M.L. Ter-Mikaelyan, Influence of the Medium on Electromagnetic Processes at High Energies (Armenian Sci. Acad., Yerevan, 1969). [7] V.N. Bajer, V.M. Katkov and V.S. Fadin, Radiation of Relativistic Electrons (Atomic Edit., Moscow, 1973) p. 234. [8] Ts. A. Amatuni, Yu. M. Antipov, S.P. Denisov and A.I. Petrukhin, Preprint IHEP 81-109 (Serpukhov, 1981).