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Enhancement of the characteristic X-ray yield from oriented crystal irradiated by high-energy electrons
Nuclear Instruments and Methods in Physics Research B 173 (2001) 142±148
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Enhancement of the characteristic X-ray yield from oriented crystal irradiated by high-energy electrons M. Andreyashkin a, M. Inoue a, H. Nakagawa, K. Yoshida a, H. Okuno b, R. Hamatsu c,*, H. Kojima c, M. Masuyama c, T. Miyakawa d, K. Umemori e, A. Potylitsin f, I. Vnukov f, Y. Takashima g, S. Anami h, A. Enomoto h, K. Furukawa h, T. Kamitani h, Y. Ogawa h, S. Ohsawa h a
Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, Higashi-Hiroshima 739-8526, Japan b High Energy Accelerator Research Organization, Tanashi-Branch, 3-2-1 Midoricho, Tanashi, Tokyo 188-8501, Japan c Department of Physics, Graduate School of Science, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachiohji, Tokyo 192-0397, Japan d Tokyo University of Agriculture and Technology, 2-24-16 Nakamachi, Koganei, Tokyo 184-0012, Japan e Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0032, Japan f Nuclear Physics Institute, Tomsk Polytechnic University, 634050, P.O. Box 25, Tomsk-50, Russia g Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan h High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba-shi 305-0801, Japan Received 7 December 1999; received in revised form 1 August 2000
Abstract In this report, results of a measurement of characteristic X-ray (CXR) yields from oriented tungsten crystal targets irradiated by 600±1000 MeV electrons are presented. Characteristic X-rays from tungsten are measured with a Si(Li) semiconductor detector placed at the backward direction with respect to the incident electron beam. We have observed an enhancement of the X-ray yield due to the K-shell ionization when the crystal axis h1 1 1i is oriented along the beam. The ratio of the K-line yield from the oriented crystal to the one from the disoriented crystal is about 1.6±1.9 for the target thickness of 1.2 mm at the electron energy of 1000 MeV. For L-line yields the enhancement is not appreciable. We demonstrated a possibility of using the orientation dependence of the CXR as a mean of aligning the crystal axis at the channeling condition to the beam. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Characteristic X-ray; Relativistic electrons; Oriented crystal
1. Introduction *
tsu).
Corresponding author. E-mail address: [email protected] (R. Hama-
Channeling radiation phenomena of high-energy electrons passing through a crystal target are interesting not only from the viewpoint of under-
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 3 8 7 - 6
M. Andreyashkin et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 142±148
lying physics but also from the viewpoint of practical applications for generation of intense photon and positron beams. We have studied a possibility to use the tungsten (W) crystal for generation of low-energy (< 40 MeV) positrons at 1.2 GeV electron energy [1,2]. Encouraged by the positive results, we have a plan to install the tungsten crystal in the positron target station of the KEK B-factory injector linac [3]. In general, for incident electrons to be channeled, the crystal axis or the crystal plane should be aligned to the electron beam within a critical angle called the Lindhard angle [4]. This angle is 1.4 mrad for a W target at the electron energy of 1000 MeV [5]. The alignment of the crystal axis to the beam direction is normally made by observing channeling radiation in the forward direction. However, sometimes it is impossible to install the photon detector in the forward direction. For example, when we use the crystal target at the positron target station of a linear accelerator, the forward space is occupied by solenoid magnets and accelerating cavities for the collection and the subsequent acceleration of produced positrons. A possible alternative method to determine the crystal axis is to measure the characteristic X-rays (CXR) emitted from the crystal target. Channeling of electrons may result in a redistribution of electron trajectories due to the string potential of the crystal axis for axial channeling or due to the plane potential for planer channeling. For negatively charged particles, the mean impact parameter decreases at the channeling condition and the ionization probability of inner-shell electrons increases [6,7]. Therefore, the CXR yield enhances at the channeling condition. For positively charged particles, a repulsive force moves the incident particles away from the crystal atoms and the ionization probability decreases, hence the CXR yield decreases. Since the angular distribution of the CXR is isotropic, its detection can be done at any angle with respect to the incident beam. This is quite desirable for locating the X-ray detector around the crystal target at the positron station of a linear accelerator. Experimentally, the CXR measurement for high-energy protons and pions was made by Bak et al. at CERN by using a Ge crystal target [8].
143
They placed a Si(Li) detector for the detection of X-rays in the backward direction. Their results clearly showed the expected behavior mentioned above. For pÿ with the momentum of 12 GeV/c, a 70% enhancement of the K-shell CXR yield was measured on the h1 1 0i axis of Ge. There is neither channeling radiation nor electromagnetic showers in case of hadron beams. For high-energy electrons, Aleinik et al. at Tomsk measured the CXR yield on the h1 0 0i axis of Ge using a 900 MeV internal beam of the synchrotron [9]. They measured a 30% enhancement of the CXR yield with a proportional counter placed at 90° with respect to the incident beam direction. In order to con®rm a possibility of using the CXR measurement for determining the crystal orientation, we have studied the CXR yield from the W crystal for electrons in the 1 GeV energy region. Generally when high-energy charged particles pass through a crystal target, an enhancement rate of the CXR yield depends on various factors such as target species, a crystallographic quality of the target, the target thickness, incident beam species and energies and the directional geometry of the X-ray detector. Especially for electrons hitting a thick target, the secondary eects due to the electromagnetic shower process play an important role depending on the detector setup. Therefore, we set our parameters of this measurement by taking into account the application of the W crystal at the positron target station of highenergy linear accelerators. 2. Experimental setup The measurement was made during the course of experiments to study the positron production eciency from the aligned crystal target using the electron beam from the 1.3 GeV electron synchrotron at Tanashi-branch of High Energy Accelerator Research Organization (KEK) [10]. An arrangement of the experimental apparatus is shown in Fig. 1. The electron beam hits the crystal target mounted on a goniometer with three rotational axes. The intensity and the duty factor of the extracted electron beam during this measurement were about 105 eÿ /s and 10%, respectively.
144
M. Andreyashkin et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 142±148
Fig. 1. Schematic layout of the experimental setup.
The beam intensity was monitored by measuring parametric X-rays from a thin (1:7 10ÿ3 X0 ) Si crystal target. The emittance of the electron beam was 2 10ÿ7 p mrad in both horizontal and vertical planes [11]. By taking into account the multiple scattering at the Si target, the FWHM angular divergence of the beam at the crystal target was estimated to be 0.9 mrad for 1000 MeV electrons and 1.45 mrad for 600 electrons. The Lindhard pMeV angle, de®ned as 2U0 =E0 , where U0 is the depth of the string potential for the h1 1 1i axis of W crystal and E0 is the electron energy, is calculated to be 1.4 and 1.8 mrad for 1000 and 600 MeV electrons, respectively [5].
For the detection of positrons emitted from the crystal target in the forward direction, a magnetic spectrometer is placed on the rotating table. In order to measure low-energy (5±20 MeV) photons due to the channeling radiation process a carbon scatterer was placed in the downstream position and the Compton-scattered photons o the carbon target were measured with an NaI detector. The energy window for the Compton-scattered photons was set to be 0.72±4.4 MeV. In addition, for the detection of high energy (50±1000 MeV) photons due to the bremsstrahlung process, crystal scintillators (NaI and CsI) were used upon request. Results on these measurements will be reported elsewhere.
M. Andreyashkin et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 142±148
145
The CXR was measured with a Si(Li) semiconductor detector located at 151° with respect to the beam direction and at the distance of 57 cm from the target as shown in Fig. 1. The active area of the detector was a disc of 16 mm in diameter and 5.7 mm in thickness. The entrance window of the detector was made of Be with a thickness of 50 lm. The energy resolution was measured to be 350 eV at 6.4 keV by using X-rays from 57 Co. 3. Results Experiments were performed twice in dierent periods using essentially the same setup. The ®rst one aimed at the observation of the CXR and the second one was to study quantitatively the enhancement of the CXR yield. 3.1. Observation of the CXR In Figs. 2(a) and (b), energy spectra of X-rays emitted by 1000 MeV electrons passing through 0.4 and 2.2 mm thick W crystals at o-axis conditions are shown. Both spectra show distinctive lines characteristic to K- and L-shell ionization of the tungsten. Even a splitting into Ka1 and Ka2 lines is clearly seen. It should be noted that the background level was very low even with no shielding and no collimator in front of the detector. Without the W target, the counting rate was negligibly small. Fig. 3(a) shows the orientation dependence of the CXR yield for the 2.2 mm thick W target together with those of the positron yield (b) and the Compton scattered photon yield (c). The energy window for X-rays was set at 56±62 keV, which included both Ka1 and Ka2 lines of W. The positron spectrometer was set at the emission angle of 0° and the momentum of 20 MeV/c. In this measurement, the goniometer was rotated around the vertical axis, whereas the horizontal axis was ®xed in the h1 1 0i plane. When the h1 1 1i axis is aligned to the electron beam, roughly a factor of 2 increase of the X-ray yield around the CXR K-line was observed. This factor is equal to or even larger than those for the positron and for the Compton-scattered photon. We obtain r 10 mrad for CXR,
Fig. 2. Energy spectrum of X rays from (a) the 0.4 mm thick and (b) the 2.2 mm thick tungsten crystals at o-axis orientation under irradiation of the 1 GeV electron beam as observed in the backward direction. Both spectra (a) and (b) are normalized to the number of electrons.
r 14 mrad for positrons and r 5 mrad for soft photons by making the curve ®tting to Gaussians. We tried to measure the orientation dependence of CXR for planer channeling conditions. In this case, we did not see any increase of the CXR yield. 3.2. Measurement of the CXR spectra For the study of the CXR characteristics, we measured detailed energy spectra from the W crystal target, when the h1 1 1i axis of the crystal was oriented along the electron beam and when disoriented. This series of measurements were made using the 1.2 mm thick W target and the electron beam with energies of 1000, 800 and 600 MeV. The yields of the Ka1 , Ka2 , La1;2 and Lb1;2
146
M. Andreyashkin et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 142±148
Fig. 4. Result of ®tting of experimental photon energy spectrum of characteristic X rays from 1.2 mm thick tungsten crystal at h1 1 1i axis orientation under irradiation of the 1GeV electron beam.
continuum. A typical ®tting result is shown in Fig. 4. In Table 1, the ratios between yields from the oriented and disoriented crystal targets are listed for each CXR line and the continuum. The experimental results show that the enhancement factor of K-lines for the oriented target is 50±90%. On the other hand, no enhancement was observed for L-lines. As for the continuum, the enhancement factor is 40±70%. Also, it should be noted that the energy dependence of the CXR enhancement is not appreciable.
Fig. 3. Crystal orientation dependence of yields of (a) characteristic X rays; (b) positrons and (c) Compton-scattered photons around the h1 1 1i axis of the 2.2 mm thick tungsten crystal.
lines were obtained separately by ®tting the spectrum with Gaussian functions with three parameters for CXR peaks and the monotonous
4. Discussions and conclusions In the ®rst measurement, we clearly observed the CXR from the W crystal as shown in Fig. 2. When we compare yields from the W crystal target with dierent thicknesses (0.4 and 2.2 mm) at oaxis conditions, yield ratios, Yield(2.2 mm)/
Table 1 Ratios between yields from the oriented and disoriented W crystal target of 1.2 mm thick Electron energy (MeV)
La
Lb
Ka2
Ka1
Continuum
1000 800 600
1:24 0:13 0:98 0:11 1:21 0:13
1:14 0:11 1:10 0:11 1:25 0:12
1:62 0:16 1:52 0:16 1:48 0:13
1:92 0:18 1:54 0:15 1:57 0:12
1:72 0:13 1:55 0:12 1:41 0:07
M. Andreyashkin et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 142±148
Yield(0.4 mm), are 1.5 and 1.3 for K- and L-lines, respectively, after subtracting the continuum. The yield ratio for the continuum region is 2.8±5.3 depending on the X-ray energy, which is roughly proportional to the target thickness. These targetthickness dependence of the CXR yield will give us some hints on the origin of the CXR. In general, CXR from a thick target hit by high-energy electrons is emitted by incident electrons, secondary photons and secondary electrons (positrons), therefore, the contribution of each process to the total CXR yield depends on the thickness of the target and on the location of the X-ray detector. When the X-ray detector is placed downstream from the target, whose thickness is greater than the absorption length of X-rays and the de-channeling length, X-rays entering the detector originate only from the back surface layer of the target. In this case, the contribution from secondary eects might be important. Especially, when the electron beam is used, the secondary effect due to the electro- and photo-ionization of atoms caused by the electromagnetic shower particles and the channeling radiation will dominate. On the other hand, when the detector is placed in the backward direction from the target, the CXR emitted from the front surface layer of the crystal is detected, because the characteristic absorption lengths in W are 10±20 lm for L- and 300±400 lm for K-rays. In this case, the initial electron and backscattered electrons(positrons) will make a contribution to the ionization and excitation of atoms. By comparing the measured CXR yields from two targets with the thickness of 0.4 and 2.2 mm, we can conclude that the dominant contribution comes from the initial electron. In the Tomsk experiment [9], the electron beam was used on the Ge target and the X-rays were detected at 90° with a proportional counter. They observed 30% enhancement. However, owing to the poor energy resolution of the detector for Xrays, it was not possible to extract the pure CXR K-line enhancement. The results of the second measurement with the 1.2 mm thick target are shown in Table 1, which indicate that the CXR L-line enhancement was not observed or small if any in contrast to the 50±90% enhancement of K-lines. The dierence in en-
147
hancement factors suggests some interpretations of the CXR phenomena from the crystal target. Under the in¯uence of the electric ®eld potential, channeled electrons are ``quasi-focused'' close to atomic strings at a certain thickness from the surface [7]. In the vicinity of this point, the impact parameters of channeled electrons become the minimum, and hence the ionization cross section is the maximum. Therefore, the above result suggests that the characteristic thickness of the CXR formation is greater than roughly 20 lm in the W target and this may result in no enhancement of CXR L-lines due to the absorption in the target material. In conclusion, we summarize our experimental results as follows: 1. The CXR emitted from the W target by GeV electrons can be clearly detected in the backward direction even for the target as thick as 2.2 mm (0:6X0 ). 2. The initial electron plays a dominant role over the back-scattered particles in emitting the CXR in the backward direction. 3. We observed the enhancement of the CXR Kline yield for the axially oriented W crystal target over that for the disoriented target. The CXR K-line enhancement factor is 1.5±1.9 for the 1.2 mm thick crystal. 4. The CXR spectrum analysis showed that the CXR enhancement was only on K-lines and no remarkable enhancement on lines. These results clearly show that the CXR measurement can be eectively used for orienting the W crystal axis along the electron beam. Acknowledgements The authors thank Prof. I. Endo of Hiroshima University for allowing us to use the Si(Li) detector for the CXR measurement and the NaI(Tl) detector system for the PXR measurement. References [1] K. Yoshida et al., Phys. Rev. Lett. 80 (1998) 1437. [2] B.N. Kalinin et al., Nucl. Instr. and Meth. B 145 (1998) 209.
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[3] K. Yoshida et al., Contribution paper to this symposium (1999). [4] J. Lindhard, K. Dan, Vid. Selsk. Mat. Fys. Medd. 34 (14) (1965). [5] V.K. Basylev, N.K. Zhevago, Radiation of Fast Particles In Matter and External Fields, Nauka, Moscow, 1987. [6] A.N. Sorensen, Nucl. Instr. and Meth. B 119 (1996) 1. [7] M.A. Kumakhov, F.F. Komarov, Radiation From Charged Particles in Solids, AIP, New York, 1989 (translation series).
[8] J.F. Bak et al., Phys. Rev. A 25 (1982) 1334. [9] A.N. Aleinik et al., Pis*ma v Zh. Tek. Fiz. T. 13 (22) (1987) 1367. [10] T. Miyakawa, Master Thesis, Tokyo University of Agriculture and Technology, 1999. [11] Y. Hashimoto et al., Annual report 75, Institute for Nuclear Study, University of Tokyo, 1995.
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Enhancement of the characteristic X-ray yield from oriented crystal irradiated by high-energy electrons M. Andreyashkin a, M. Inoue a, H. Nakagawa, K. Yoshida a, H. Okuno b, R. Hamatsu c,*, H. Kojima c, M. Masuyama c, T. Miyakawa d, K. Umemori e, A. Potylitsin f, I. Vnukov f, Y. Takashima g, S. Anami h, A. Enomoto h, K. Furukawa h, T. Kamitani h, Y. Ogawa h, S. Ohsawa h a
Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, Higashi-Hiroshima 739-8526, Japan b High Energy Accelerator Research Organization, Tanashi-Branch, 3-2-1 Midoricho, Tanashi, Tokyo 188-8501, Japan c Department of Physics, Graduate School of Science, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachiohji, Tokyo 192-0397, Japan d Tokyo University of Agriculture and Technology, 2-24-16 Nakamachi, Koganei, Tokyo 184-0012, Japan e Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0032, Japan f Nuclear Physics Institute, Tomsk Polytechnic University, 634050, P.O. Box 25, Tomsk-50, Russia g Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan h High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba-shi 305-0801, Japan Received 7 December 1999; received in revised form 1 August 2000
Abstract In this report, results of a measurement of characteristic X-ray (CXR) yields from oriented tungsten crystal targets irradiated by 600±1000 MeV electrons are presented. Characteristic X-rays from tungsten are measured with a Si(Li) semiconductor detector placed at the backward direction with respect to the incident electron beam. We have observed an enhancement of the X-ray yield due to the K-shell ionization when the crystal axis h1 1 1i is oriented along the beam. The ratio of the K-line yield from the oriented crystal to the one from the disoriented crystal is about 1.6±1.9 for the target thickness of 1.2 mm at the electron energy of 1000 MeV. For L-line yields the enhancement is not appreciable. We demonstrated a possibility of using the orientation dependence of the CXR as a mean of aligning the crystal axis at the channeling condition to the beam. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Characteristic X-ray; Relativistic electrons; Oriented crystal
1. Introduction *
tsu).
Corresponding author. E-mail address: [email protected] (R. Hama-
Channeling radiation phenomena of high-energy electrons passing through a crystal target are interesting not only from the viewpoint of under-
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 3 8 7 - 6
M. Andreyashkin et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 142±148
lying physics but also from the viewpoint of practical applications for generation of intense photon and positron beams. We have studied a possibility to use the tungsten (W) crystal for generation of low-energy (< 40 MeV) positrons at 1.2 GeV electron energy [1,2]. Encouraged by the positive results, we have a plan to install the tungsten crystal in the positron target station of the KEK B-factory injector linac [3]. In general, for incident electrons to be channeled, the crystal axis or the crystal plane should be aligned to the electron beam within a critical angle called the Lindhard angle [4]. This angle is 1.4 mrad for a W target at the electron energy of 1000 MeV [5]. The alignment of the crystal axis to the beam direction is normally made by observing channeling radiation in the forward direction. However, sometimes it is impossible to install the photon detector in the forward direction. For example, when we use the crystal target at the positron target station of a linear accelerator, the forward space is occupied by solenoid magnets and accelerating cavities for the collection and the subsequent acceleration of produced positrons. A possible alternative method to determine the crystal axis is to measure the characteristic X-rays (CXR) emitted from the crystal target. Channeling of electrons may result in a redistribution of electron trajectories due to the string potential of the crystal axis for axial channeling or due to the plane potential for planer channeling. For negatively charged particles, the mean impact parameter decreases at the channeling condition and the ionization probability of inner-shell electrons increases [6,7]. Therefore, the CXR yield enhances at the channeling condition. For positively charged particles, a repulsive force moves the incident particles away from the crystal atoms and the ionization probability decreases, hence the CXR yield decreases. Since the angular distribution of the CXR is isotropic, its detection can be done at any angle with respect to the incident beam. This is quite desirable for locating the X-ray detector around the crystal target at the positron station of a linear accelerator. Experimentally, the CXR measurement for high-energy protons and pions was made by Bak et al. at CERN by using a Ge crystal target [8].
143
They placed a Si(Li) detector for the detection of X-rays in the backward direction. Their results clearly showed the expected behavior mentioned above. For pÿ with the momentum of 12 GeV/c, a 70% enhancement of the K-shell CXR yield was measured on the h1 1 0i axis of Ge. There is neither channeling radiation nor electromagnetic showers in case of hadron beams. For high-energy electrons, Aleinik et al. at Tomsk measured the CXR yield on the h1 0 0i axis of Ge using a 900 MeV internal beam of the synchrotron [9]. They measured a 30% enhancement of the CXR yield with a proportional counter placed at 90° with respect to the incident beam direction. In order to con®rm a possibility of using the CXR measurement for determining the crystal orientation, we have studied the CXR yield from the W crystal for electrons in the 1 GeV energy region. Generally when high-energy charged particles pass through a crystal target, an enhancement rate of the CXR yield depends on various factors such as target species, a crystallographic quality of the target, the target thickness, incident beam species and energies and the directional geometry of the X-ray detector. Especially for electrons hitting a thick target, the secondary eects due to the electromagnetic shower process play an important role depending on the detector setup. Therefore, we set our parameters of this measurement by taking into account the application of the W crystal at the positron target station of highenergy linear accelerators. 2. Experimental setup The measurement was made during the course of experiments to study the positron production eciency from the aligned crystal target using the electron beam from the 1.3 GeV electron synchrotron at Tanashi-branch of High Energy Accelerator Research Organization (KEK) [10]. An arrangement of the experimental apparatus is shown in Fig. 1. The electron beam hits the crystal target mounted on a goniometer with three rotational axes. The intensity and the duty factor of the extracted electron beam during this measurement were about 105 eÿ /s and 10%, respectively.
144
M. Andreyashkin et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 142±148
Fig. 1. Schematic layout of the experimental setup.
The beam intensity was monitored by measuring parametric X-rays from a thin (1:7 10ÿ3 X0 ) Si crystal target. The emittance of the electron beam was 2 10ÿ7 p mrad in both horizontal and vertical planes [11]. By taking into account the multiple scattering at the Si target, the FWHM angular divergence of the beam at the crystal target was estimated to be 0.9 mrad for 1000 MeV electrons and 1.45 mrad for 600 electrons. The Lindhard pMeV angle, de®ned as 2U0 =E0 , where U0 is the depth of the string potential for the h1 1 1i axis of W crystal and E0 is the electron energy, is calculated to be 1.4 and 1.8 mrad for 1000 and 600 MeV electrons, respectively [5].
For the detection of positrons emitted from the crystal target in the forward direction, a magnetic spectrometer is placed on the rotating table. In order to measure low-energy (5±20 MeV) photons due to the channeling radiation process a carbon scatterer was placed in the downstream position and the Compton-scattered photons o the carbon target were measured with an NaI detector. The energy window for the Compton-scattered photons was set to be 0.72±4.4 MeV. In addition, for the detection of high energy (50±1000 MeV) photons due to the bremsstrahlung process, crystal scintillators (NaI and CsI) were used upon request. Results on these measurements will be reported elsewhere.
M. Andreyashkin et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 142±148
145
The CXR was measured with a Si(Li) semiconductor detector located at 151° with respect to the beam direction and at the distance of 57 cm from the target as shown in Fig. 1. The active area of the detector was a disc of 16 mm in diameter and 5.7 mm in thickness. The entrance window of the detector was made of Be with a thickness of 50 lm. The energy resolution was measured to be 350 eV at 6.4 keV by using X-rays from 57 Co. 3. Results Experiments were performed twice in dierent periods using essentially the same setup. The ®rst one aimed at the observation of the CXR and the second one was to study quantitatively the enhancement of the CXR yield. 3.1. Observation of the CXR In Figs. 2(a) and (b), energy spectra of X-rays emitted by 1000 MeV electrons passing through 0.4 and 2.2 mm thick W crystals at o-axis conditions are shown. Both spectra show distinctive lines characteristic to K- and L-shell ionization of the tungsten. Even a splitting into Ka1 and Ka2 lines is clearly seen. It should be noted that the background level was very low even with no shielding and no collimator in front of the detector. Without the W target, the counting rate was negligibly small. Fig. 3(a) shows the orientation dependence of the CXR yield for the 2.2 mm thick W target together with those of the positron yield (b) and the Compton scattered photon yield (c). The energy window for X-rays was set at 56±62 keV, which included both Ka1 and Ka2 lines of W. The positron spectrometer was set at the emission angle of 0° and the momentum of 20 MeV/c. In this measurement, the goniometer was rotated around the vertical axis, whereas the horizontal axis was ®xed in the h1 1 0i plane. When the h1 1 1i axis is aligned to the electron beam, roughly a factor of 2 increase of the X-ray yield around the CXR K-line was observed. This factor is equal to or even larger than those for the positron and for the Compton-scattered photon. We obtain r 10 mrad for CXR,
Fig. 2. Energy spectrum of X rays from (a) the 0.4 mm thick and (b) the 2.2 mm thick tungsten crystals at o-axis orientation under irradiation of the 1 GeV electron beam as observed in the backward direction. Both spectra (a) and (b) are normalized to the number of electrons.
r 14 mrad for positrons and r 5 mrad for soft photons by making the curve ®tting to Gaussians. We tried to measure the orientation dependence of CXR for planer channeling conditions. In this case, we did not see any increase of the CXR yield. 3.2. Measurement of the CXR spectra For the study of the CXR characteristics, we measured detailed energy spectra from the W crystal target, when the h1 1 1i axis of the crystal was oriented along the electron beam and when disoriented. This series of measurements were made using the 1.2 mm thick W target and the electron beam with energies of 1000, 800 and 600 MeV. The yields of the Ka1 , Ka2 , La1;2 and Lb1;2
146
M. Andreyashkin et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 142±148
Fig. 4. Result of ®tting of experimental photon energy spectrum of characteristic X rays from 1.2 mm thick tungsten crystal at h1 1 1i axis orientation under irradiation of the 1GeV electron beam.
continuum. A typical ®tting result is shown in Fig. 4. In Table 1, the ratios between yields from the oriented and disoriented crystal targets are listed for each CXR line and the continuum. The experimental results show that the enhancement factor of K-lines for the oriented target is 50±90%. On the other hand, no enhancement was observed for L-lines. As for the continuum, the enhancement factor is 40±70%. Also, it should be noted that the energy dependence of the CXR enhancement is not appreciable.
Fig. 3. Crystal orientation dependence of yields of (a) characteristic X rays; (b) positrons and (c) Compton-scattered photons around the h1 1 1i axis of the 2.2 mm thick tungsten crystal.
lines were obtained separately by ®tting the spectrum with Gaussian functions with three parameters for CXR peaks and the monotonous
4. Discussions and conclusions In the ®rst measurement, we clearly observed the CXR from the W crystal as shown in Fig. 2. When we compare yields from the W crystal target with dierent thicknesses (0.4 and 2.2 mm) at oaxis conditions, yield ratios, Yield(2.2 mm)/
Table 1 Ratios between yields from the oriented and disoriented W crystal target of 1.2 mm thick Electron energy (MeV)
La
Lb
Ka2
Ka1
Continuum
1000 800 600
1:24 0:13 0:98 0:11 1:21 0:13
1:14 0:11 1:10 0:11 1:25 0:12
1:62 0:16 1:52 0:16 1:48 0:13
1:92 0:18 1:54 0:15 1:57 0:12
1:72 0:13 1:55 0:12 1:41 0:07
M. Andreyashkin et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 142±148
Yield(0.4 mm), are 1.5 and 1.3 for K- and L-lines, respectively, after subtracting the continuum. The yield ratio for the continuum region is 2.8±5.3 depending on the X-ray energy, which is roughly proportional to the target thickness. These targetthickness dependence of the CXR yield will give us some hints on the origin of the CXR. In general, CXR from a thick target hit by high-energy electrons is emitted by incident electrons, secondary photons and secondary electrons (positrons), therefore, the contribution of each process to the total CXR yield depends on the thickness of the target and on the location of the X-ray detector. When the X-ray detector is placed downstream from the target, whose thickness is greater than the absorption length of X-rays and the de-channeling length, X-rays entering the detector originate only from the back surface layer of the target. In this case, the contribution from secondary eects might be important. Especially, when the electron beam is used, the secondary effect due to the electro- and photo-ionization of atoms caused by the electromagnetic shower particles and the channeling radiation will dominate. On the other hand, when the detector is placed in the backward direction from the target, the CXR emitted from the front surface layer of the crystal is detected, because the characteristic absorption lengths in W are 10±20 lm for L- and 300±400 lm for K-rays. In this case, the initial electron and backscattered electrons(positrons) will make a contribution to the ionization and excitation of atoms. By comparing the measured CXR yields from two targets with the thickness of 0.4 and 2.2 mm, we can conclude that the dominant contribution comes from the initial electron. In the Tomsk experiment [9], the electron beam was used on the Ge target and the X-rays were detected at 90° with a proportional counter. They observed 30% enhancement. However, owing to the poor energy resolution of the detector for Xrays, it was not possible to extract the pure CXR K-line enhancement. The results of the second measurement with the 1.2 mm thick target are shown in Table 1, which indicate that the CXR L-line enhancement was not observed or small if any in contrast to the 50±90% enhancement of K-lines. The dierence in en-
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hancement factors suggests some interpretations of the CXR phenomena from the crystal target. Under the in¯uence of the electric ®eld potential, channeled electrons are ``quasi-focused'' close to atomic strings at a certain thickness from the surface [7]. In the vicinity of this point, the impact parameters of channeled electrons become the minimum, and hence the ionization cross section is the maximum. Therefore, the above result suggests that the characteristic thickness of the CXR formation is greater than roughly 20 lm in the W target and this may result in no enhancement of CXR L-lines due to the absorption in the target material. In conclusion, we summarize our experimental results as follows: 1. The CXR emitted from the W target by GeV electrons can be clearly detected in the backward direction even for the target as thick as 2.2 mm (0:6X0 ). 2. The initial electron plays a dominant role over the back-scattered particles in emitting the CXR in the backward direction. 3. We observed the enhancement of the CXR Kline yield for the axially oriented W crystal target over that for the disoriented target. The CXR K-line enhancement factor is 1.5±1.9 for the 1.2 mm thick crystal. 4. The CXR spectrum analysis showed that the CXR enhancement was only on K-lines and no remarkable enhancement on lines. These results clearly show that the CXR measurement can be eectively used for orienting the W crystal axis along the electron beam. Acknowledgements The authors thank Prof. I. Endo of Hiroshima University for allowing us to use the Si(Li) detector for the CXR measurement and the NaI(Tl) detector system for the PXR measurement. References [1] K. Yoshida et al., Phys. Rev. Lett. 80 (1998) 1437. [2] B.N. Kalinin et al., Nucl. Instr. and Meth. B 145 (1998) 209.
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[3] K. Yoshida et al., Contribution paper to this symposium (1999). [4] J. Lindhard, K. Dan, Vid. Selsk. Mat. Fys. Medd. 34 (14) (1965). [5] V.K. Basylev, N.K. Zhevago, Radiation of Fast Particles In Matter and External Fields, Nauka, Moscow, 1987. [6] A.N. Sorensen, Nucl. Instr. and Meth. B 119 (1996) 1. [7] M.A. Kumakhov, F.F. Komarov, Radiation From Charged Particles in Solids, AIP, New York, 1989 (translation series).
[8] J.F. Bak et al., Phys. Rev. A 25 (1982) 1334. [9] A.N. Aleinik et al., Pis*ma v Zh. Tek. Fiz. T. 13 (22) (1987) 1367. [10] T. Miyakawa, Master Thesis, Tokyo University of Agriculture and Technology, 1999. [11] Y. Hashimoto et al., Annual report 75, Institute for Nuclear Study, University of Tokyo, 1995.