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Study on the transient field at very high velocities for the g-factor measurement of excited states in unstable nuclei
ELSEWER
Nuclear Physic? A738 (2004) 519-522 www.elsevier.com/locate/npe
Study on the transient field at very high velocities for the g-factor measurement of excited states in unstable nuclei A. Yoshinii", H. Ueno". W. &tob. H. Watanabe", Y. Kobayashi". J . Illurata", H. Miyoshid, K. Shimadad, and K.Asahiad "RIKEN. Hirosawa 2-1, Wako-shi, Saitama 351-0198, Japan bDepartment of Chemistry, Osaka University, Machikaneyamacho 1-1. Toyonah-shi, Osaka 560-0043, Japan "Department of Physics, Rikkyo University, Nishi-Ikebukuro 3-34-1, Toshima-ku, Tokyo 171-8501, Japan dDepartment of Physics, Tokyo Institute of Technology, Oh-okayama 2-12-1, Meguro-ku, Tokyo 152-8551, Japan
For the g-factor measurements of short-lived unstable nuclei, we have developed a setup for the transient field experiment, at RIKEK. In order t,o carry out the g-factor measurement, we need to know the st,rengh of the transient. field at high beam velocities. For this purpose, our setup was used to measure the transient filed act,ing on 22Se ions passing through the Gd layer at a beam velocity of v = 14v0. We found that, the obt,ained strength BTF= 2.4(8) kT is much larger than the empirical expectation. 1. INTRODUCTION Based on the ,&XMR method with the fragment-induced spin-polarized radioaetiveisotope (RI) beams, t’heground-state nuclear mornent,s have been measured in the region of nuclei far from the stability line [1-4]. From the obtained results, t,he effects of increasing neutron number on their structure have been discussed. Static nuclear moments, however, can not, be measured in principle from the ground state of even-even unstable nuclei. One of such unstable nuclei is 3251g: for which intriguing anomalies are observed [5]. In order t’o extract electromagnetic informat,ion from such even-even riuclei, measurements of nuclear moments for short-lived excited states are needed. For the g-fact>ormeasimment of thc excited states in unst,able nuclei, we have developed a set,up for t,ransient-field (TF) experiment. It is well known that t,he ions moving in polarized ferromagnetic targets experience an effective magnetic field, so called transient field, as st,rong as several kT [6]. The TF technique has been applied to the measurement of the g-factors of short-lived nuclea,r stat>esat only low beam velocities. However, taking into account the fact t.hat RI beams produced in the projectile fragmentatmionreaction have very high velocities compared t o the 1s-electron Bohr velocity v l S = %VO (wo = c/137), it, is import,ant to know the TF strength at that velocity [7,8]. Theoretically, 0375-94741s see front matter 8 2004 Elsevier B.V All rights reserved doi: 10. I0 I6/j.nuclphysa.2004.04.10 I -
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A. Yoshinti et ul. /Nuclear Physics A738 (2004) 519-522
no successful quantitative description of the TF from first principles has been given so far. The earlier empirical models lead t o the expectation of a strong decrease of TF as the velocity increases. whereas the recent model predicts the rather constant or even increasing trend of the TF with increasing velocity [9]. To investigate this experimentally, the nieasurement of the transient-field strength has been carried out at the velocity of 2: = 14vo using a "Ne beam. 2. EXPERIMENTAL PROCEDURE The setup for the TF experiments is schematically shown in Fig. 1. A target consisting of Au and Gd foils facing each other was placed in the vacuum chamber, which was cooled to the liquid N2 temperature in order t o obtain the saturation magnetization. The thickness of the target foils, Au and Gd. werr 21.4 and 6.4 mg/cm2 respectively. An external magnetic field of Bo = 0.045 T was applied perpendicular to the beam axis using a couple of coils placed outside the chamber. The Bo direction was periodically changed up and down every 15 seconds alternately for reducing the systematic error. A beam of 22Nefrom k = 70 RIKEN AVF cyclotron was used to bombard the target. The averaged beam energy E = 4.9 AMeV in the Gd layer corresponds to a beam velocity 2’ = 1 4 ~ 0During . the experiment, the beam current was controlled to be 1 electric-nil. The J" = 2+ first excited state of 22Neat Ex = 1.275 McV. whose magnetic monient and lifetime are known to be p = +0.65(2) pr and 7 = 3.63(5) ps. respectively, was populated in thc Au layer through the Coulomb excitation. The n/ rays emitted froin the cxcited state were detected with four KaI(T1) detectors placed at the angles 0lab = 553" and 5100" relativ? to the incoming beam axis.
-
Vacuum Chamber
Figure 1. A schematic layout of the det,ector system for the TF experiment.
Segmented plastic scintillators, consisting of 30 plastic scintillators mounted cylindrically 7.5 cm from the beam axis, were installed inside the vacuum chamber as fast-response
A . Yoshiini et al. /Nuclear Physics A738 (2004) 519-522
521
particle counters, with which the de-excitat,iony-rays can be measured in coincidence with the sca.ttered beam particle with a high S/N ra,tio.
3. RESULTS AND DISCUSSION The de-excitation y-rays from the first excited state of "Ne were identified as a peak in the y spectrum shown in Fig. 2 , where the background is considerably reduced compared with our previous experiment [lo]. With the obtained y-ray spectrum, a particle--/ angular correlation was measured. Figure 3 (a) shows the obtained angular distribution as a
Figure 2. A y-ray spectrum obt
function of the y-ray emission angle. The detector system also allows us to derive a detailed particle-y angular correlation as a function of azimut,h angle $, of the scattered particle in a similar manner described in Ref. [7]. Figure 3 (b) shows the particle-y angular correlation determined from the photo-peak counting ratios measured with the pair of NaI detectors located symmetric to the beam axis.
Figure 3 . The particle-angular correlation as a function of (a) the y-ray emission angle and (b) t,he azimuth angle of t,he scattered part,icle.
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A . Yoshinzi et al. /Nuclew Physics A738 (2004) 519-522
In the case that the precession angle of the nuclear moment is small, the TF strength BTFis approximately given by
where PLR is the double ratio of photo-peak counting rates measured with the pair of NaI detectors under Bo in the up and down directions, g the g-factor of the "Ne(2+) statme, tetf the interaction time of the Ne beam with the TF, B the yemission angle from the beam axis, and w(8) the angular distribution of 7 ray. Thus, we have determined the TF strength BTF= 2.4(8) kT. We also confirmed that the effect is caused not by the direction change of t,he BOfield but by the TF, since no TF effect is found in the value BTF= 0.7(10) kT that was obt,ained without the Gd layer. The obtained TF strength is valuable for understanding the mechanism of TF. The present data supports the mechanism proposed in Ref. [9], where the TF is considered t,o be produccd by the spin exchange process between the s-electrons of t,he incident ions and a number of polarized electrons of tBheferromagnetic host. In the proposed theory in Ref. [9], the TF strength remains coiist,ant,or even increasing trend with the ion velocity beyond vls. The observed BTF,and also the T F strength observed in Ref. [8], BTF = 1.9(8) kT at the 20Ne-beam velocity v = 1 2 . 5 ~ in 0 the Gd target, seem t,o suggest t,his tendency.
4. CONCLUSION For the g-fador measurements of excited states of RI beams, we have developed a detector system for T F experiments. In the experiment with 22Nebeam. t>hede-excit,at#ion :/-rays from t,he first excit,ed state and their detailed particle-y correlation were measured with a high S/N rat,io. Owing to this, the TF strength at the beam velocity of 11 = 14v0 was determined as BTF= 2.4(8) kT. This result indicates that t#heTF strength at t>his velocit#yis even larger in contrast to the empirical expectation. The higher accuracy of the BTFvalues, however: is needed in the detailed discussion concerning the behavior of TF strengt,h and its mechanism at the high bean1 velocities.
REFERENCES 1. H. Ueno et al., Phys. Rev. C 53 (1996) 2142. 2. H. Ixumi et al., Phys. Lett. B 366 (1996) 51. 3. H. Ogawa et al.. Phys. Lett. B 451 (1999) 11. 4. H. Ogawa et al., Phys. Rev. C 67 (2003) 064308. 5. T. Motobayashi et al., Phys. Lett. B 346 (1995) 9. 6. R.R. Borchers et al., Phys. Rev. Lett. 20 (1968) 424. 7. J. Cub et al., Nucl. Phys. A 549 (1992) 304. 8. K.-H. Speidel et al.. Z. Phys. 339 (1991) 265. 9. F. Hagelbcrg et nl., Phys. Rev. C 48 (1993) 2230. 10. H. Ueno et al., Hyperfine hit. 136 (2001) 211.
Nuclear Physic? A738 (2004) 519-522 www.elsevier.com/locate/npe
Study on the transient field at very high velocities for the g-factor measurement of excited states in unstable nuclei A. Yoshinii", H. Ueno". W. &tob. H. Watanabe", Y. Kobayashi". J . Illurata", H. Miyoshid, K. Shimadad, and K.Asahiad "RIKEN. Hirosawa 2-1, Wako-shi, Saitama 351-0198, Japan bDepartment of Chemistry, Osaka University, Machikaneyamacho 1-1. Toyonah-shi, Osaka 560-0043, Japan "Department of Physics, Rikkyo University, Nishi-Ikebukuro 3-34-1, Toshima-ku, Tokyo 171-8501, Japan dDepartment of Physics, Tokyo Institute of Technology, Oh-okayama 2-12-1, Meguro-ku, Tokyo 152-8551, Japan
For the g-factor measurements of short-lived unstable nuclei, we have developed a setup for the transient field experiment, at RIKEK. In order t,o carry out the g-factor measurement, we need to know the st,rengh of the transient. field at high beam velocities. For this purpose, our setup was used to measure the transient filed act,ing on 22Se ions passing through the Gd layer at a beam velocity of v = 14v0. We found that, the obt,ained strength BTF= 2.4(8) kT is much larger than the empirical expectation. 1. INTRODUCTION Based on the ,&XMR method with the fragment-induced spin-polarized radioaetiveisotope (RI) beams, t’heground-state nuclear mornent,s have been measured in the region of nuclei far from the stability line [1-4]. From the obtained results, t,he effects of increasing neutron number on their structure have been discussed. Static nuclear moments, however, can not, be measured in principle from the ground state of even-even unstable nuclei. One of such unstable nuclei is 3251g: for which intriguing anomalies are observed [5]. In order t’o extract electromagnetic informat,ion from such even-even riuclei, measurements of nuclear moments for short-lived excited states are needed. For the g-fact>ormeasimment of thc excited states in unst,able nuclei, we have developed a set,up for t,ransient-field (TF) experiment. It is well known that t,he ions moving in polarized ferromagnetic targets experience an effective magnetic field, so called transient field, as st,rong as several kT [6]. The TF technique has been applied to the measurement of the g-factors of short-lived nuclea,r stat>esat only low beam velocities. However, taking into account the fact t.hat RI beams produced in the projectile fragmentatmionreaction have very high velocities compared t o the 1s-electron Bohr velocity v l S = %VO (wo = c/137), it, is import,ant to know the TF strength at that velocity [7,8]. Theoretically, 0375-94741s see front matter 8 2004 Elsevier B.V All rights reserved doi: 10. I0 I6/j.nuclphysa.2004.04.10 I -
520
A. Yoshinti et ul. /Nuclear Physics A738 (2004) 519-522
no successful quantitative description of the TF from first principles has been given so far. The earlier empirical models lead t o the expectation of a strong decrease of TF as the velocity increases. whereas the recent model predicts the rather constant or even increasing trend of the TF with increasing velocity [9]. To investigate this experimentally, the nieasurement of the transient-field strength has been carried out at the velocity of 2: = 14vo using a "Ne beam. 2. EXPERIMENTAL PROCEDURE The setup for the TF experiments is schematically shown in Fig. 1. A target consisting of Au and Gd foils facing each other was placed in the vacuum chamber, which was cooled to the liquid N2 temperature in order t o obtain the saturation magnetization. The thickness of the target foils, Au and Gd. werr 21.4 and 6.4 mg/cm2 respectively. An external magnetic field of Bo = 0.045 T was applied perpendicular to the beam axis using a couple of coils placed outside the chamber. The Bo direction was periodically changed up and down every 15 seconds alternately for reducing the systematic error. A beam of 22Nefrom k = 70 RIKEN AVF cyclotron was used to bombard the target. The averaged beam energy E = 4.9 AMeV in the Gd layer corresponds to a beam velocity 2’ = 1 4 ~ 0During . the experiment, the beam current was controlled to be 1 electric-nil. The J" = 2+ first excited state of 22Neat Ex = 1.275 McV. whose magnetic monient and lifetime are known to be p = +0.65(2) pr and 7 = 3.63(5) ps. respectively, was populated in thc Au layer through the Coulomb excitation. The n/ rays emitted froin the cxcited state were detected with four KaI(T1) detectors placed at the angles 0lab = 553" and 5100" relativ? to the incoming beam axis.
-
Vacuum Chamber
Figure 1. A schematic layout of the det,ector system for the TF experiment.
Segmented plastic scintillators, consisting of 30 plastic scintillators mounted cylindrically 7.5 cm from the beam axis, were installed inside the vacuum chamber as fast-response
A . Yoshiini et al. /Nuclear Physics A738 (2004) 519-522
521
particle counters, with which the de-excitat,iony-rays can be measured in coincidence with the sca.ttered beam particle with a high S/N ra,tio.
3. RESULTS AND DISCUSSION The de-excitation y-rays from the first excited state of "Ne were identified as a peak in the y spectrum shown in Fig. 2 , where the background is considerably reduced compared with our previous experiment [lo]. With the obtained y-ray spectrum, a particle--/ angular correlation was measured. Figure 3 (a) shows the obtained angular distribution as a
Figure 2. A y-ray spectrum obt
function of the y-ray emission angle. The detector system also allows us to derive a detailed particle-y angular correlation as a function of azimut,h angle $, of the scattered particle in a similar manner described in Ref. [7]. Figure 3 (b) shows the particle-y angular correlation determined from the photo-peak counting ratios measured with the pair of NaI detectors located symmetric to the beam axis.
Figure 3 . The particle-angular correlation as a function of (a) the y-ray emission angle and (b) t,he azimuth angle of t,he scattered part,icle.
522
A . Yoshinzi et al. /Nuclew Physics A738 (2004) 519-522
In the case that the precession angle of the nuclear moment is small, the TF strength BTFis approximately given by
where PLR is the double ratio of photo-peak counting rates measured with the pair of NaI detectors under Bo in the up and down directions, g the g-factor of the "Ne(2+) statme, tetf the interaction time of the Ne beam with the TF, B the yemission angle from the beam axis, and w(8) the angular distribution of 7 ray. Thus, we have determined the TF strength BTF= 2.4(8) kT. We also confirmed that the effect is caused not by the direction change of t,he BOfield but by the TF, since no TF effect is found in the value BTF= 0.7(10) kT that was obt,ained without the Gd layer. The obtained TF strength is valuable for understanding the mechanism of TF. The present data supports the mechanism proposed in Ref. [9], where the TF is considered t,o be produccd by the spin exchange process between the s-electrons of t,he incident ions and a number of polarized electrons of tBheferromagnetic host. In the proposed theory in Ref. [9], the TF strength remains coiist,ant,or even increasing trend with the ion velocity beyond vls. The observed BTF,and also the T F strength observed in Ref. [8], BTF = 1.9(8) kT at the 20Ne-beam velocity v = 1 2 . 5 ~ in 0 the Gd target, seem t,o suggest t,his tendency.
4. CONCLUSION For the g-fador measurements of excited states of RI beams, we have developed a detector system for T F experiments. In the experiment with 22Nebeam. t>hede-excit,at#ion :/-rays from t,he first excit,ed state and their detailed particle-y correlation were measured with a high S/N rat,io. Owing to this, the TF strength at the beam velocity of 11 = 14v0 was determined as BTF= 2.4(8) kT. This result indicates that t#heTF strength at t>his velocit#yis even larger in contrast to the empirical expectation. The higher accuracy of the BTFvalues, however: is needed in the detailed discussion concerning the behavior of TF strengt,h and its mechanism at the high bean1 velocities.
REFERENCES 1. H. Ueno et al., Phys. Rev. C 53 (1996) 2142. 2. H. Ixumi et al., Phys. Lett. B 366 (1996) 51. 3. H. Ogawa et al.. Phys. Lett. B 451 (1999) 11. 4. H. Ogawa et al., Phys. Rev. C 67 (2003) 064308. 5. T. Motobayashi et al., Phys. Lett. B 346 (1995) 9. 6. R.R. Borchers et al., Phys. Rev. Lett. 20 (1968) 424. 7. J. Cub et al., Nucl. Phys. A 549 (1992) 304. 8. K.-H. Speidel et al.. Z. Phys. 339 (1991) 265. 9. F. Hagelbcrg et nl., Phys. Rev. C 48 (1993) 2230. 10. H. Ueno et al., Hyperfine hit. 136 (2001) 211.