SEVENTH ALL-UNION CONFERENCE ON THE

OF INTERACTION OF CHARGED PARTICLES

WITH SINGLE CRYSTALS

A. G. K a d m e n s k i i

P H Y S I C S

The Conference took place in Moscow on May 26-28, 1975. The topic of d iscuss ion at this and the p reced ing confe rences were the re la t ive ly recent ly d i scovered effects of channeling and shadowing in the mot ion of f a s t charged p a r t i c l e s in single c r y s t a l s and phenomena assoc ia ted with these effects . The Con-" fe rence was attended by near ly 200 sc ien t i s t s of var ious Soviet c i t ies who rep resen ted un ivers i t i e s and r e s e a r c h inst i tutes of the Academy of Sciences of the USSR and o ther Soviet Republics, and also s eve ra l m i n i s t r i e s and depa r tmen t s . P a p e r s have also been p resen ted by prominent sc ien t i s t s f r o m the USA, GDR, Poland, Mongolia, and Yugoslavia . The 93 r epo r t s read at the Conference touched upon var ious a spec t s of this rapidly developing subject . All r epo r t s can be a r b i t r a r i l y divided into th ree m a j o r groups. F i rs t , the study of or ienta t ional ef fec ts of the mot ion of heavy charged pa r t i c l e s (protons, ions) in single c rys ta l s ; second, works assoc ia ted with the mot ion of light charged pa r t i c l e s (electrons, pos i t rons) and radia t ion in single c ry s t a l s ; and thi rd , the appl icat ion of or ientat ional effects in the study of nuclear , atomic, solid s ta te , and o ther p r o b l e m s in single c r y s t a l s . Such a c lass i f i ca t ion takes into account not only the kind of pa r t i c le but a l so the different f o r m of descr ip t ion of the mot ion of heavy and light pa r t i c l e s in single c r y s t a l s . Af te r the work of J . Linhard (1965) on the channeling of heavy pa r t i c l e s , the bas ic model of de- sc r ip t ion is the c l a s s i ca l model of s ta t i s t i ca l equi l ibr ium in the phase space of t r a n s v e r s e coordinates and pa r t i c l e m o m e n t a in the c rys t a l (the amount of t r a n s v e r s a l i t y re la t ive to the d i rec t ion of a tomic nucleus for axial channeling and re la t ive to the a tomic plane fo r p l ana r channeling). Such a descr ip t ion can also be used for h igh-energy e lec t rons but the main approach to this case is a quantum mechanica l t r ea tmen t .

The study of channel ing and shadowing ef fec ts of heavy p a r t i c l e s now p roceeds in two di rec t ions . The method of kinetic equations fo r channeling pa r t i c l e s , developed on the bas i s of the Linhard model by Danish (Banderoop, Andersen, and others) and Soviet (M. A. Kumakhov, V. V. Beloshitski i , et al.) worke r s , is ex- tens ive ly used fo r calcula t ing dechanneling and the e lements of angular shadow-pa t te rn dis t r ibut ions. Such calculat ions have been repor ted by, for example , M. A. Kumakhov, V. V. Beloshitski i , et al. (Moscow) and by V. S. Andreev e t a l . (Sverdl0vsk). On the other hand, exper imenta l and theore t ica l r e su l t s have been r e - por ted which indicate that axial and p lana r channeling are assoc ia ted phenomena. Thus, the p reva len t fo r - m a l i s m of kinetic equations does not desc r ibe ce r t a in effects of pa r t i c l e channeling. Important work on this subject has been c a r r i e d out by Yu. V. Bulgakov, V. I. Shul 'ga et al . (Moscow) who studied the behavior of c r y s t a l t r a n s p a r e n c y in the t rans i t ion region between axial and p l ana r channeling where the t r anspa rency has been exper imenta l ly obse rved to vary nonmonotonically with the angle of pa r t i c le incidence in re la t ion to the c lose -packed axis . This effect has been explained by the p r e s e n c e of additional focusing by o rdered a tomic chains of the par t i c le moving in the axial channel.

The o rde red a r r a n g e m e n t of a tomic chains is mani fes ted exper imenta l ly in the azimuthal dependence of the e las t ic s ca t t e r ing c r o s s sec t ion of channeling pa r t i c l e s in the axial channel ("back shadow"). This has been mos t c l ea r ly demons t ra ted in a study of the spec t r a of b a c k - s c a t t e r e d pa r t i c l e s enter ing an axial channel with the s ame t r a n s v e r s e energy (E• 0.5E• r, where E tc r is the Lindhard c r i t i ca l energy), but with different s t a r t ing az imuths [E. I. Sirotinin, A. S. Rudnev, et al. (Moscow)].

Finally, compute r s imula t ion of angular dis t r ibut ions of pa r t i c l e s pass ing through a thin single c r y s - tal under axial channeling conditions [A. G. Kadmenski i et al. (Moscow)] proved that fo r E l 0 -> 0.2 E• r angular d is t r ibut ions show a r egu la r azimuthal s t ruc tu re that coincides with the pr inc ipa l d i rec t ion of c rys ta l lograph ic planes pa s s ing th rough the studied c rys ta l lograph ic axis . It has been found that these angular

Trans la t ed f r o m A t o m n a y a t~nergiya, Vol. 39, No. 6, pp. 443-444, December , 1975.

~ @19 76 Plenum Publishing Corporation, 22 7 West 17th Street, New York, N. Y. t 001 t. No part o f this publication may be reproduced, ]s tored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, ....recording or otherwise, without written permission .of the publisher. A copy of this article is available from the publisher for $15.00.

1111

distr ibut ions a re fa r f rom being in s ta t i s t ica l equi l ibr ium in the Lindhard sense, and that the t r a n s v e r s e energy dis tr ibut ion also differs f r o m the predic t ions of the Lindhard model of s ta t i s t ica l equi l ibr ium for an axial channel.

All these fac ts will undoubtedly s t imula te furthe r development of the theory of or ientat ional phenomena.

Considerable p r o g r e s s has b e e n m a d e in the important p rob lem of the m e c h a n i s m s and magnitude of energy loss by channeling ions. A. F. Burenkov et al. (Moscow, Minsk) calculated the energy loss of a channeling par t ic le taking into account the band s t ruc tu re of the e lec t ron energy spec t rum in c rys t a l s . The rat io of the energy loss in motion through an axial channel to the energy loss of a par t ic le moving in a nonoriented d i rec t ion has been found to be a nonmonotonic function of the par t ic le energy and is e s s e n - t ial ly different f rom 1/2 as p r ed i c t edby the Lindhard rule of equipart i t ion of energy lo s ses into distant and nea r col l is ions. An exper imenta l study of average re la t ive energy losses for var ious tungsten axes and planes , c a r r i ed out by the th i ck - t a rge t method [E. I. Sirotinin, A. S.Rudnev, A. F. Tulinov et al. Moscow ], has found that for all or ienta t ions the f igures a re ~ 1.5 and ~0.35, respect ive ly , for c ry s t a l s escaping f rom the sur face l a y e r s and f rom g r e a t e r depths.

Channeling of e lec t rons and pos i t rons is act ively studied in Tomsk, Kharkov, and Moscow. Of p a r - t i cu la r in teres t a re a t tempts to analyze the bound state of an e lec t ron moving in channeling conditions, and the b r e m s s t r a h l u n g of such an e lec t ron in the p lanar and axial cases iN. 1). Kalashnikov, et al. (Moscow); S. A. Vorob 'ev , V. V. Kaplin, e t al. (Tomsk)].

The field of applicat ion of orientat ional effects i nc reases f r o m y e a r to yea r . Widely known are the resu l t s of m e a s u r e m e n t s of the l i fe t ime of exci ted nuclei in nuclear react ions in single c rys t a l t a rge t s , and the local izat ion of impur i ty a toms in the c rys ta l l ine lat t ice. Yu. V. Melikov, P. E. Vorotnikov, N. G. Chechenin, et al. (Moscow) repor ted on the m e a s u r e m e n t of ~ U f i ss ion t ime as a function of exci tat ion energy in a wide range of ene rg ies . The obtained resu l t s indicate that the level density of the studied nucleus has in the invest igated energy range a discontinuity which cannot be explained by exis t ing theo- re t ica l models . Considerable advances have been made in the de te rmina t ion of the si te of impur i t i es (oxy- gen, carbon) in semiconductor c ry s t a l s by Yu. Yu. Kryuchkov, N. V. Slavin, et al. (Tomsk), who used high- energy ~ pa r t i c l e s .

The calculat ion of the th ree -d imens iona l dis t r ibut ion of par t i c les in a channel and of i ts va r ia t ion with depth makes it poss ib le now to find the depth distr ibution of impuri ty ions and defects in a c rys t a l by the method of par t ic le backsca t te r ing . In t e r e s t i ngwork in this d i rec t ion has been jointly descr ibed by a group of Soviet and GDR sc ient i s t s .

Invest igat ions of the dynamics of phase t rans i t ions in single c ry s t a l s and of the s t ruc tu re of he te ro - epitaxial f i lms [A. A. Puzanov, et al. (Sverdlovsk)] proved that the applicat ion of charged par t i c le beams in such studies can provide new information, which grea t ly s u r p a s s e s the capabi l i t ies of t radi t ional meth- ods (e. g., x - r a y methods) .

New poss ib i l i t i es in a tomic studies have been d iscovered by German sc ien t i s t s (H. Otto, et al.). They succeeded in recording together with a pro ton diffract ion pat tern , a c i rc le represen t ing the c ro s s sect ion of the Kossel cone of the cha rac t e r i s t i c x - r a y radiat ion of the c rys t a l a toms . Of specia l in teres t is the new approach to the study of dynamic in teract ion of ions with the aid of channeling which has been put forward by D. Hammel (USA). If a col l imated beam of mo lecu la r H2 + ions en te r s a thin single c rys t a l in the d i rec t ion of the c lose -packed axis, the molecule d is in tegra tes inside the c rys ta l , but protons con- tinue to move in a co r re l a t ed f o r m in adjacent channels . The angular and energy distr ibut ions of pa r t i c les escaping the c rys t a l c a r r y important informat ion about the in terac t ion of such pro tons with the medium.

1112