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Spatial distribution of γ-radiation from electrons in aligned single crystals
Physics Letters A 174 (1993) 169-173 North-Holland
PHYSICS LETTERS A
Spatial distribution of T-radiation from electrons in aligned single crystals M.Yu. A n d r e y a s h k i n , S.A. V o r o b i e v *, V.N. Z a b a e v , A.P. Kashirin, G.A. N a u m e n k o , A.P. Potylitsin a n d V.M. T a r a s o v Nuclear Physics Institute, Tomsk Polytechnical University, P.O. Box 25, 634050 Tomsk, Russian Federation
Received 22 May 1992; revised manuscript received 4 January 1993; accepted for publication 6 January 1993 Communicatedby L.J. Sham
The measurements of gamma-radiationangular distribution of 900 MeV electrons in aligned diamond and silicon crystalsare presented. For the first time the difference in behaviour of the hard and soft fractions of gamma-radiation in aligned and misaligned crystalswas studied experimentally.
1. Introduction The channeling radiation (CR) of charged relativistic particles in single crystals, discovered in the 70s, has a principal advantage over the bremsstrahlung (BS) in amorphous radiation. This concerns its high intensity under axial channeling, narrow directivity, high polarization degree in the planar case, quasimonochromaticity, etc. These merits have given rise to extensive experimental and theoretical studies of this effect over the last decade. Numerous works, however, reported the spectral CR characteristics by averaging over the photon ejection angle 0v in the range of a given T-beam collimation. Meanwhile, it has been found that the electron transmission through a crystal is followed by a considerable spatial redistribution of the initial particle beam which leads to the dependence of the spatial Tquantum composition on the observation angle 0v [ 1 ]. There lies the difference from electron bremsstrahlung. It seems necessary to measure the photon energy and its angle of ejection from a target in order to interpret the experimental data and to obtain an agreement with the theory. The aim is to ascertain different mechanisms of electron emission, to determine * ProfessorS.A. Vorobievdied on 8 January 1992. Elsevier SciencePublishers B.V.
optimum areas of CR applications and to study the dynamics of the electron transmission through crystals. This paper presents the experimental results of spectral-angular characteristics of T-radiation emitted by electrons under axial channeling and at angles of crystal axis disalignment comparable with the critical channeling angle ~,. For measurements we used the Tomsk synchrotron with internal electron beam. (Previous measured results were reported in ref. [2].) The accelerating electron energy was 900 MeV, the energy monochromaticity AEo/Eo-~0.5% and the duration of the electron incidence at the target r ~ 10-20 ms. The angular divergence of the electron beam was as low a s 10 - 4 rad.
2. Experimental equipment The schematics of the experiment is shown in fig. 1. Accelerated electrons strike an internal monocrystalline target (T) placed into a goniometer. The T-radiation beam passes through a collimator (Col) and upon clearing from charged particles by a magnetic system (SM) reaches the experimental hall through a vacuum line. In a converter (PI), the photons are partially converted into pairs which cross a 16 9
Volume 174, number 1,2
1 March 1993
PHYSICS LETTERS A C
~
L.--',.~J ~
v///////////,~
Fig. 1. Experimental scheme.
set of spark wire chambers (Ch) and a thin scintillation counter (C), and fall down into a totalabsorption y-spectrometer. It is based on a NaI (T1) single crystal 200 × 200 m m 2 in size with a resolution AWl W~_ 9% for 6°Co "/-lines and a linearity within A W = 2 - 3 0 0 MeV. The spectrometer aperture was restricted by the collimator and attained 4.4 and 8 mrad in the horizontal and vertical planes, respectively. Note that the radiation angle of electrons with energy E o = 9 0 0 MeV is significantly smaller, mc2/Eo~_0.6 mrad. The spark chamber (Ch) was used to detect particles in the cone with the angle 0 = 7.5 mrad. The spatial resolution o f the Ch was 0.25 mm. In our case this corresponds to 2 × 10 -2 mrad. The efficiency of the ~/-quanta conversion for an e + / e - pair in a 0.5 m m thick lead converter was 3-4%. The thin scintillation counter (C) of 5 m m thickness produced vapours in combination with initial "~quanta. Due to ionization loss of the pair particles in the converter and the scintillation counter, the detection threshold of the "/-quanta in the experiment was W t h ~--- 6-8 MeV. The current o f the accelerated particles was monitored by induction current gauge and synchrotron radiation gauge [ 3 ] techniques. 3. S p e c t r a l and angular c h a r a c t e r i s t i c s o f y-radiation
Earlier [4] we measured the spectra o f y-radiation 170
for various axially aligned single crystals. Figure 2 shows the intensity spectra in absolute units. For these measurements the collimation angle was 0c = 0.6 mrad. The intensity is normalized to an electron incident on a target. Every spectrum shown in fig. 2 has a m a x i m u m in the soft part. It is clear that the anomalous part of the CR intensity spectrum, corresponding to the radiation of the electrons captured in a regime of bound and quasibound motion, is in the "/-energy range W < 80 MeV. The hard spectrum part which is mainly determined by the emission of above-barrier electrons ( W > 120 MeV) has the same shape as in the case o f bremsstrahlung in disaligned crystals.
.~0 ~0"j
~.o ..~0-~ 5.
"~.
2o~
~3oi •
• io
"%..-.L':';::-:-..:-.: v',-
20;
-,.'." - :.'- -,-..
"~:%o. " 0%0
,
4.0
". "::o ~-
"° ° ° o
I
I
":~ 0
~20
,° t° • " ° 1~•
:, ~0, HeY
200
o;.~.
" " i °" ° *1o• * °~1 °%
i
uJ, MeV Fig. 2. The intensity spectra of y-emission from electrons in various single crystals with collimation angle 0~= 0.6 mrad. ( 1 ) Diamond, t=0.35 mm, (110) axis; (2) tungsten, t=0.64 mm,
( 111 ); (3) Ge, t=0.75 mm, ( 100); (4) Si, t=0.35 mm, ( 100); (5) Si, t= 10 ram, (100).
Volume 174, number 1,2
PHYSICS LETTERSA
The spectral and angular CR characteristics were measured using a 0.35 m m thick diamond crystal with ( 1 1 0 ) axis alignment and a 0.37 m m thick silicon crystal with ( 1 0 0 ) axis alignment. For quantitative analysis of the angular photon distribution we used the mean-square deviation a. The statistical error in determining a is as low as 6-8%. Table 1 lists the coefficients of the relative changes in the widths of angular distributions of T-quanta in the soft and hard regions, r/=tr< >/e~d, where e< > and grand indicate the mean-square deviations of the angular photon distributions under axial and random crystal alignment, respectively. It is clear from the table that upon switching from random to axial alignment the soft part of the spectrum becomes narrower, while the hard part is broadened. We can give the following interpretation for such a fundamental difference in the behavior of these two fractions of T-radiation. (1) In a randomly aligned crystal, the angular photon distribution of any energy value is determined by two components: the electron radiation angle itself y - 1 and the angle of multiple scattering of electrons in the target 0o. Then Orand~ X//~0 + 1/y 2 . (2) In an axially aligned crystal, the soft photons belonging to the CR peak region are emitted by electrons captured into the channel and the low-lying ones above the barrier levels. In this case, the electron motion results from the interaction between electrons and joint potential of atomic chains, the critical channeling angle ~L being the measure of the width of the angular photon distribution. Since in our experiments ~L < 00 (we used diamond and silicon crystals with Eo=900 MeV, ~ L ( d i a m o n d ) ~ ~L(Si) ~0.45 mrad; 0o=0.8 mrad) the angular distribution in the soft fraction of T-radiation becomes Table 1 Energy range (MeV)
Target diamond
silicon
<80 120-900 120-300
0.74 1.31
0.75 1.42 -
1 March 1993
narrower upon switching from random to axial crystal alignment. (3) Photons which are responsible for the "tail" of the intensity spectrum are emitted by the abovebarrier electrons. Therefore, the broadening in the angular distribution of hard photons indicates that under axial channeling the above-barrier electron scattering becomes stronger due to a joint potential of axis as compared to the case of disaligned target. That broadening of the hard component of Tradiation in silicon crystal (t/= 1.42) corresponds to an increased angle 00 (~/= 1.5) for the electron energy of 1200 MeV observed in Kharkov [5 ].
4. T-beam parameters at crystal alignment angles comparable with q/L A difference in the angular distribution of soft and hard photons is also observed for crystal axis disalignment angles comparable with the critical channeling angle. Figures 3a-3c show the cross sections of the T-beam for three alignments of the ( 1 1 0 ) diamond crystal axis with respect to the primary electron beam ~ = 0 , 0.8 and 1.5 mrad. We can see the T-beam direction follows the deviation of the crystal axis. In this case the hard photon section ( W= 80300 MeV) shows a ring structure. The ring radius is seen to correlate with the crystal axis disalignment angle. With further crystal disalignment the ring structure disappears. For an energy W< 80 MeV (fig. 3d), the direction of the T-beam is similarly shifted. However, there is no ring structure in the T-beam cross section. For a quantitative analysis of the T-beam rotation with respect to the primary electron beam let us determine the displacement of the gravity centre ( G C ) of the photon angular distribution. The displacement of the GC at the ( 1 0 0 ) axis, for a rotation of the silicon crystal, was A=0.5~, with ~v< 1 mrad. This is consistent with the data reported in ref. [ 6 ]. The diamond crystal shows a larger value of the displacement, A=0.75, with V< 1 mrad. With further crystal disalignment the GC returned to the original position. The effect observed testifies to the fact that the electron beam becomes sensitive to the joint potential of the crystal axis, when the alignment angles ex171
Volume 174, number 1,2
PHYSICS LETTERS A
1 March 1993
~i::!ii ¸ , i
:
9-
1
I
/;~,~:+:
~:~
I
L
c
I
--4
!
':9
.... l . . . . . . . ~ . t
.........
0
~: :: :~:iil;':!ii:!!:
::L:=::J
5
: : ~ i ............................... : . . . .
~3
"2
"I
O
I
2~
5
:, mr d
Fig. 3. The T-beam profile at different rotation angles of diamond crystal over the horizontal axis; ( + ) indicates the spatial direction of the primary electron beam; ( × ) t h e crystal axis direction; (a) ~Uh=0 (axial channeling); (b) q/h=0.8 mrad; (c) ~,h= 1.5 mrad; (d) qJh= 1 mrad.
ceed the critical channeling angle by a factor o f 2.
5. Conclusion F r o m the above results we can deduce that the b r o a d e n i n g o f the h a r d fraction o f 7-emission at axial channeling and the "/-beam direction rotation following the rotation o f the crystal are governed by the same mechanism, viz. the interaction o f the joint axis potential with the above-barrier electrons. 172
The authors o f ref. [ 6 ] stated that the angular Tb e a m distribution near the axial alignment is indep e n d e n t of the T-quanta energy. In our experiment we observed an essential difference in the b e h a v i o u r o f the angular distribution in the soft a n d hard sections of the spectrum. The angular distribution o f hard photons shows a well p r o n o u n c e d a s y m m e t r y which appears in the case o f crystal alignment near the axis a n d must lead, as was shown in ref. [ 6], to polarization o f y-radiation within this energy range. This necessitates the
Volume 174, number 1,2
PHYSICS LETTERS A
c o n t i n u a t i o n o f the r e s e a r c h o f the a b o v e effects, w h i c h m i g h t l e a d to c r e a t i o n o f a n e w s o u r c e o f highenergy p o l a r i z e d y - q u a n t a .
References
1 March 1993
[2] M.Yu. Andreyashkin et al., in: Abstracts IVth All-Union Conference on Interaction of radiation with solids (Moscow, 1990) p. 129. [3 ] M.Yu. Andreyashkin et al., Prib. Tekh. Eksp. 6 (1989 ) 55. [4 ] M.Yu. Andreyashkin et al., J. Nuel. Phys. 53 (1991 ) 335. [ 5 ] A.P. Antipenko et al., in; Proc. XVIII All-Union Conference on Physics of interaction of charged particles with crystals (Moscow, 1989) p. 96. [6] I. Endo et al., preprint HUPD-9117 ( 1991 ).
[ I ] M.Yu. Andreyashkin et al., JETP Lett. 53 ( 1991 ) 497.
173
PHYSICS LETTERS A
Spatial distribution of T-radiation from electrons in aligned single crystals M.Yu. A n d r e y a s h k i n , S.A. V o r o b i e v *, V.N. Z a b a e v , A.P. Kashirin, G.A. N a u m e n k o , A.P. Potylitsin a n d V.M. T a r a s o v Nuclear Physics Institute, Tomsk Polytechnical University, P.O. Box 25, 634050 Tomsk, Russian Federation
Received 22 May 1992; revised manuscript received 4 January 1993; accepted for publication 6 January 1993 Communicatedby L.J. Sham
The measurements of gamma-radiationangular distribution of 900 MeV electrons in aligned diamond and silicon crystalsare presented. For the first time the difference in behaviour of the hard and soft fractions of gamma-radiation in aligned and misaligned crystalswas studied experimentally.
1. Introduction The channeling radiation (CR) of charged relativistic particles in single crystals, discovered in the 70s, has a principal advantage over the bremsstrahlung (BS) in amorphous radiation. This concerns its high intensity under axial channeling, narrow directivity, high polarization degree in the planar case, quasimonochromaticity, etc. These merits have given rise to extensive experimental and theoretical studies of this effect over the last decade. Numerous works, however, reported the spectral CR characteristics by averaging over the photon ejection angle 0v in the range of a given T-beam collimation. Meanwhile, it has been found that the electron transmission through a crystal is followed by a considerable spatial redistribution of the initial particle beam which leads to the dependence of the spatial Tquantum composition on the observation angle 0v [ 1 ]. There lies the difference from electron bremsstrahlung. It seems necessary to measure the photon energy and its angle of ejection from a target in order to interpret the experimental data and to obtain an agreement with the theory. The aim is to ascertain different mechanisms of electron emission, to determine * ProfessorS.A. Vorobievdied on 8 January 1992. Elsevier SciencePublishers B.V.
optimum areas of CR applications and to study the dynamics of the electron transmission through crystals. This paper presents the experimental results of spectral-angular characteristics of T-radiation emitted by electrons under axial channeling and at angles of crystal axis disalignment comparable with the critical channeling angle ~,. For measurements we used the Tomsk synchrotron with internal electron beam. (Previous measured results were reported in ref. [2].) The accelerating electron energy was 900 MeV, the energy monochromaticity AEo/Eo-~0.5% and the duration of the electron incidence at the target r ~ 10-20 ms. The angular divergence of the electron beam was as low a s 10 - 4 rad.
2. Experimental equipment The schematics of the experiment is shown in fig. 1. Accelerated electrons strike an internal monocrystalline target (T) placed into a goniometer. The T-radiation beam passes through a collimator (Col) and upon clearing from charged particles by a magnetic system (SM) reaches the experimental hall through a vacuum line. In a converter (PI), the photons are partially converted into pairs which cross a 16 9
Volume 174, number 1,2
1 March 1993
PHYSICS LETTERS A C
~
L.--',.~J ~
v///////////,~
Fig. 1. Experimental scheme.
set of spark wire chambers (Ch) and a thin scintillation counter (C), and fall down into a totalabsorption y-spectrometer. It is based on a NaI (T1) single crystal 200 × 200 m m 2 in size with a resolution AWl W~_ 9% for 6°Co "/-lines and a linearity within A W = 2 - 3 0 0 MeV. The spectrometer aperture was restricted by the collimator and attained 4.4 and 8 mrad in the horizontal and vertical planes, respectively. Note that the radiation angle of electrons with energy E o = 9 0 0 MeV is significantly smaller, mc2/Eo~_0.6 mrad. The spark chamber (Ch) was used to detect particles in the cone with the angle 0 = 7.5 mrad. The spatial resolution o f the Ch was 0.25 mm. In our case this corresponds to 2 × 10 -2 mrad. The efficiency of the ~/-quanta conversion for an e + / e - pair in a 0.5 m m thick lead converter was 3-4%. The thin scintillation counter (C) of 5 m m thickness produced vapours in combination with initial "~quanta. Due to ionization loss of the pair particles in the converter and the scintillation counter, the detection threshold of the "/-quanta in the experiment was W t h ~--- 6-8 MeV. The current o f the accelerated particles was monitored by induction current gauge and synchrotron radiation gauge [ 3 ] techniques. 3. S p e c t r a l and angular c h a r a c t e r i s t i c s o f y-radiation
Earlier [4] we measured the spectra o f y-radiation 170
for various axially aligned single crystals. Figure 2 shows the intensity spectra in absolute units. For these measurements the collimation angle was 0c = 0.6 mrad. The intensity is normalized to an electron incident on a target. Every spectrum shown in fig. 2 has a m a x i m u m in the soft part. It is clear that the anomalous part of the CR intensity spectrum, corresponding to the radiation of the electrons captured in a regime of bound and quasibound motion, is in the "/-energy range W < 80 MeV. The hard spectrum part which is mainly determined by the emission of above-barrier electrons ( W > 120 MeV) has the same shape as in the case o f bremsstrahlung in disaligned crystals.
.~0 ~0"j
~.o ..~0-~ 5.
"~.
2o~
~3oi •
• io
"%..-.L':';::-:-..:-.: v',-
20;
-,.'." - :.'- -,-..
"~:%o. " 0%0
,
4.0
". "::o ~-
"° ° ° o
I
I
":~ 0
~20
,° t° • " ° 1~•
:, ~0, HeY
200
o;.~.
" " i °" ° *1o• * °~1 °%
i
uJ, MeV Fig. 2. The intensity spectra of y-emission from electrons in various single crystals with collimation angle 0~= 0.6 mrad. ( 1 ) Diamond, t=0.35 mm, (110) axis; (2) tungsten, t=0.64 mm,
( 111 ); (3) Ge, t=0.75 mm, ( 100); (4) Si, t=0.35 mm, ( 100); (5) Si, t= 10 ram, (100).
Volume 174, number 1,2
PHYSICS LETTERSA
The spectral and angular CR characteristics were measured using a 0.35 m m thick diamond crystal with ( 1 1 0 ) axis alignment and a 0.37 m m thick silicon crystal with ( 1 0 0 ) axis alignment. For quantitative analysis of the angular photon distribution we used the mean-square deviation a. The statistical error in determining a is as low as 6-8%. Table 1 lists the coefficients of the relative changes in the widths of angular distributions of T-quanta in the soft and hard regions, r/=tr< >/e~d, where e< > and grand indicate the mean-square deviations of the angular photon distributions under axial and random crystal alignment, respectively. It is clear from the table that upon switching from random to axial alignment the soft part of the spectrum becomes narrower, while the hard part is broadened. We can give the following interpretation for such a fundamental difference in the behavior of these two fractions of T-radiation. (1) In a randomly aligned crystal, the angular photon distribution of any energy value is determined by two components: the electron radiation angle itself y - 1 and the angle of multiple scattering of electrons in the target 0o. Then Orand~ X//~0 + 1/y 2 . (2) In an axially aligned crystal, the soft photons belonging to the CR peak region are emitted by electrons captured into the channel and the low-lying ones above the barrier levels. In this case, the electron motion results from the interaction between electrons and joint potential of atomic chains, the critical channeling angle ~L being the measure of the width of the angular photon distribution. Since in our experiments ~L < 00 (we used diamond and silicon crystals with Eo=900 MeV, ~ L ( d i a m o n d ) ~ ~L(Si) ~0.45 mrad; 0o=0.8 mrad) the angular distribution in the soft fraction of T-radiation becomes Table 1 Energy range (MeV)
Target diamond
silicon
<80 120-900 120-300
0.74 1.31
0.75 1.42 -
1 March 1993
narrower upon switching from random to axial crystal alignment. (3) Photons which are responsible for the "tail" of the intensity spectrum are emitted by the abovebarrier electrons. Therefore, the broadening in the angular distribution of hard photons indicates that under axial channeling the above-barrier electron scattering becomes stronger due to a joint potential of axis as compared to the case of disaligned target. That broadening of the hard component of Tradiation in silicon crystal (t/= 1.42) corresponds to an increased angle 00 (~/= 1.5) for the electron energy of 1200 MeV observed in Kharkov [5 ].
4. T-beam parameters at crystal alignment angles comparable with q/L A difference in the angular distribution of soft and hard photons is also observed for crystal axis disalignment angles comparable with the critical channeling angle. Figures 3a-3c show the cross sections of the T-beam for three alignments of the ( 1 1 0 ) diamond crystal axis with respect to the primary electron beam ~ = 0 , 0.8 and 1.5 mrad. We can see the T-beam direction follows the deviation of the crystal axis. In this case the hard photon section ( W= 80300 MeV) shows a ring structure. The ring radius is seen to correlate with the crystal axis disalignment angle. With further crystal disalignment the ring structure disappears. For an energy W< 80 MeV (fig. 3d), the direction of the T-beam is similarly shifted. However, there is no ring structure in the T-beam cross section. For a quantitative analysis of the T-beam rotation with respect to the primary electron beam let us determine the displacement of the gravity centre ( G C ) of the photon angular distribution. The displacement of the GC at the ( 1 0 0 ) axis, for a rotation of the silicon crystal, was A=0.5~, with ~v< 1 mrad. This is consistent with the data reported in ref. [ 6 ]. The diamond crystal shows a larger value of the displacement, A=0.75, with V< 1 mrad. With further crystal disalignment the GC returned to the original position. The effect observed testifies to the fact that the electron beam becomes sensitive to the joint potential of the crystal axis, when the alignment angles ex171
Volume 174, number 1,2
PHYSICS LETTERS A
1 March 1993
~i::!ii ¸ , i
:
9-
1
I
/;~,~:+:
~:~
I
L
c
I
--4
!
':9
.... l . . . . . . . ~ . t
.........
0
~: :: :~:iil;':!ii:!!:
::L:=::J
5
: : ~ i ............................... : . . . .
~3
"2
"I
O
I
2~
5
:, mr d
Fig. 3. The T-beam profile at different rotation angles of diamond crystal over the horizontal axis; ( + ) indicates the spatial direction of the primary electron beam; ( × ) t h e crystal axis direction; (a) ~Uh=0 (axial channeling); (b) q/h=0.8 mrad; (c) ~,h= 1.5 mrad; (d) qJh= 1 mrad.
ceed the critical channeling angle by a factor o f 2.
5. Conclusion F r o m the above results we can deduce that the b r o a d e n i n g o f the h a r d fraction o f 7-emission at axial channeling and the "/-beam direction rotation following the rotation o f the crystal are governed by the same mechanism, viz. the interaction o f the joint axis potential with the above-barrier electrons. 172
The authors o f ref. [ 6 ] stated that the angular Tb e a m distribution near the axial alignment is indep e n d e n t of the T-quanta energy. In our experiment we observed an essential difference in the b e h a v i o u r o f the angular distribution in the soft a n d hard sections of the spectrum. The angular distribution o f hard photons shows a well p r o n o u n c e d a s y m m e t r y which appears in the case o f crystal alignment near the axis a n d must lead, as was shown in ref. [ 6], to polarization o f y-radiation within this energy range. This necessitates the
Volume 174, number 1,2
PHYSICS LETTERS A
c o n t i n u a t i o n o f the r e s e a r c h o f the a b o v e effects, w h i c h m i g h t l e a d to c r e a t i o n o f a n e w s o u r c e o f highenergy p o l a r i z e d y - q u a n t a .
References
1 March 1993
[2] M.Yu. Andreyashkin et al., in: Abstracts IVth All-Union Conference on Interaction of radiation with solids (Moscow, 1990) p. 129. [3 ] M.Yu. Andreyashkin et al., Prib. Tekh. Eksp. 6 (1989 ) 55. [4 ] M.Yu. Andreyashkin et al., J. Nuel. Phys. 53 (1991 ) 335. [ 5 ] A.P. Antipenko et al., in; Proc. XVIII All-Union Conference on Physics of interaction of charged particles with crystals (Moscow, 1989) p. 96. [6] I. Endo et al., preprint HUPD-9117 ( 1991 ).
[ I ] M.Yu. Andreyashkin et al., JETP Lett. 53 ( 1991 ) 497.
173