Coulomb excitation of 174Hf K-isomer. γ-ray spectroscopy with high-spin isomer beam

11 May 1995 PHYSICS LETTERS B

Physics Letters B 350 (1995) 169-172

EISEVIER

Coulomb excitation of 174HfK-isomer. y-ray spectroscopy with high-spin isomer beam T. Morikawaa, Y. Gono b, K. Morita ‘, T. Kishida c, T. Murakami d, E. Ideguchi b, H. Kumagai ‘, G.H. Liu ‘, A. Ferragut c, A. Yoshida c, Y.H. Zhang e, M. Oshima a, M. Sugawara f, H. Kusakarig, M. Ogawah, M. Nakajima h, H. Tsuchida h, S. Mitarai b, A. Odahara b, M. Kiderab, M. Shibatab, J.C. Kim’, S.J. Chae’, Y. Hatsukawaa, M. IshiharaC a Japan Atomic Energy Research Institute (JAERI), Ibaraki 319-I 1, Japan b Department of Physics, Kyushu University, Fukuoka 812, Japan c The Institute of Physical and Chemical Research (RIKEN). Saitama 351-01, Japan * National Institute of Radiological Science. Chiba 263, Japan e Institute of Modem Physics, Lanzhou, China ’ Chiba Institute of Technology, Chiba 275, Japan g Faculty of Education, Chiba University, Chiba 263, Japan h Tokyo Institute of Technology, Kanagawa 227, Japan i Seoul National lJniversi@, Seoul 151-742, South Korea

Received 17 November 1994; revised manuscript received 7 March 1995 Editor: R.H. Siemssen

Abstract A new experimental technique utilizing a high-spin isomer beam (HSIB) has been developed. The HSIB of ‘74Hf was produced by the ‘Be( “‘Er, 5n)‘74Hf reaction in inverse kinematics. By using the HSIB, the Coulomb excitation of the K” = 8- isomer in 174Hfwas measured. The y-rays from the 9- level, which is the first excited state built on the isomer, were observed as well as the cascade y-rays of the ground-state rotational band. From the measured y-ray yield, a value of 2 f 1 e*b* was extracted for the B( E2 : 8- + 9-). This is the first experiment of y-ray spectroscopy by means of the HSIB-induced secondary reactions.

In recent years the use of secondary beams has been at the center of interest in the field of nuclear spectroscopy and it is a rapidly developing field. It is believed to be very powerful for the study of nuclei under extreme conditions which cannot be attained by other beams. Recently, a new experimental technique was developed [ 1] by utilizing a high-spin isomer as a secondary beam. Its application to y-ray spectroscopy by Coulomb excitation (COULEX) was successfully carried out by using a beam of the KT = 8- iso-

mer in 174Hf. This paper reports the first results of the COULEX experiment together with a brief introduction of the High Spin Isomer Beam (HSIB) facility at RIKEN. The HSIB experiment described here provides an unique and novel opportunity for high-spin spectroscopy. The HSIB facility at RIKEN consists of a dipole and two pairs of quadrupole magnets. A production target is bombarded by a primary beam of heavy ions. By using inverse kinematics the primary reaction is

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chosen to produce the high-spin isomer efficiently. A major part of the primary beam is intercepted by slits placed between two pairs of quadrupole magnets. The reaction products are delivered, as a secondary beam, to the secondary target position. Since the present system is not capable of isotope separation, the secondary beam is a mixture of a) the high spin isomer, b) its ground state and c) other reaction products. In this letter, however, we emphasize that it was possible to extract the transition probability from the COULEX of the HSIB by using various coincidence conditions. Up to now, HSJB of l”Prn (27 fi, > 2 ps) [ 21, 14%rn (49/2 fi, 0.96 ps> [ 3,4], 147Gd (49/2 fL,O.56 ps) [ 51 and 174Hf (8 ti, 2.4 ,us) [ 61 have been produced. Fusion experiments to produce cold high-spin compound nuclei are also planned, which are expected to populate efficiently very high-spin states [ 11. The beam of the 174Hf K” = 8- isomer was produced by the 9Be ( 170Er, 5n) ‘74mHf reaction. A primary beam of 17’Er ions was provided by the accelerator complex of a linear accelerator injector and a ring cyclotron at RIKEN. The beam energy was 7.0 MeV/u. A production target of 1.84 mg/cm2 9Be foil was bombarded by the primary beam with a beam intensity of l-2 pnA. In the present experiment, the primary beam and the HSIB could not be fully separated due to the small difference in their magnetic rigidities. Therefore, a major part of the HSIB was lost by the slits together with the primary beam. Consequently, the 174mHf HSIB intensity was limited to be 1.5-3 x lo3 pps at the secondary target position, while the t70Er primary beam intensity was still larger than that of the HSIB by more than one order of magnitude. The HSIB intensity was extracted from the yield of characteristic gamma-rays deexciting the isomer. The beam spot size was less than 10 mm& The HSIB of 174mHf bombarded a 208Pb foil of 2 mg/cm*. Eight HPGe detectors collimated by lead shields were set at 8 = 135” with respect to the beam axis. Four delay-line PPAC’s of a triangular shape were arranged to form a pyramidal shape to detect the projectile scattered in the angular range of 15”-65” in the laboratory frame. Twenty-four NaI( Tl) scintillatom were placed surrounding the PPAC chamber to detect the delayed y-rays which deexcite the isomer following the prompt COULEX reaction. A plastic scintillator of 0.2 mm thickness was used as a catcher at the apex of the pyramid PPAC. Two HPGe’s and one

Letters B 350 (1995) 169-l 72

250L

r , 100 ,

2w

,

, 83w,

1 1 84w, r

a / 5w, 7,

(c) BackGround-Subtracted 200 > d iz

150

50 0 1w

200

300

400

500

Energy (keV) Fig. 1. Doppler-corrected y-ray spectra gated by the peak of reaction products (a) and the other region (b) in the TOF spectrum. The spectrum (c) is made by subtracting (b) from (a) with an appropnate normalization.

clover detector were located to detect y-rays from the catcher to estimate the HSIB intensity. The data acquisition system consisting of a Micro VAX and CAMAC modules was triggered by a coincidence signal between the PPAC and, at least, one of the eight Ge detectors with a 500 ns time window. The digitized data of PPAC, Ge, NaI(T1) and RF were recorded event by event on digital audio tapes. The Ge detectors were calibrated with y-ray sources of 15*Eu and ‘33Ba. The list-mode data were sorted off-line to extract the COULEX events of 174Hf isomers. The time of flight (TOF) information between the RF and the PPAC’s signals was used to separate the reaction products from the primary beam. Fig. l(a) shows a Dopplercorrected y-ray spectrum made by gating on the TOF peak of the reaction products. A y-ray spectrum of the

T. Morikawa et al. /Physics

I<== 8-, 2.4 ps 2028.0 p= 8+, 133 ns 1737.4 7+

: i j

; : i

8+

5.5

900

F

Id, 1797.5 4

8T

1549.3;

171

Letters B 350 (1995) 169-l 72

T

4.5 “0 Nz 3.5

700 2 3

3

2.5

500 == 2

X

1.5 0.5

300 x

w* 2

100 6

10

14

Q,(b) Fig. 3. B( E2) values as a function of Qo. Solid lines and dashed lines correspond to the rotors of K = 8 and K = 0, respectively. The filled square corresponds to Qo = 7.3 b and K = 0, i.e. the GSB in 174Hf, while the filled circle is taken from the present work. 207

Fig. 2. A partial level scheme of ‘74Hf with the observed transitions indicated as a solid line. Dashed arrows are the transitions following the K” = 8- isomer decay in 174Hf.

ground state band (GSB) in “OEr was obtained by setting a gate on the corresponding portion in the TOF spectrum as shown in Fig. 1 (b) . This spectrum was subtracted from that of Fig. 1 (a) with an appropriate normalization factor. In the resulting y-ray spectrum (Fig. 1 (c) ), a peak of the 9- + X- transition feeding the isomer was clearly observed at 232 keV, as well as the rotational sequence of the GSB in 174Hf. From a y-ray spectrum made by additionally gating on the delayed part of the NaI(Tl> timing, it was confirmed that the 232 keV y-ray in Fig. 1 (c) corresponded to the 9- -+ 8- transition above the K” = 8- isomer in 174Hf. In contrast, no peak was observed at a y-ray energy of 481 keV which corresponds to the lo- --f 8transition. Fig. 2 summarizes the levels and transitions observed in the present experiment. The excitation cross sections were deduced from the y-ray yields by assuming that the emission of the yrays is isotropic. The 8- + 9- excitation was treated as a pure E2 transition since the Ml ‘excitation’ was estimated to be far below the detection limit. However, in the 9- -+ 8- transition, the Ml ‘decay’ may

not be negligible; the difference in internal conversion coefficient affects the extracted cross section. In the present analysis, the 9- -+ 8- transition is treated as a pure E2 transition. In the extreme case that the transition were ‘pure Ml’, the present analysis would underestimate the cross section by about 20 %. A ratio of the excitation cross sections ~r(8- + 9-) /a( 0: 4 2g’) was extracted from the -y-ray yields by using the following relation: a(8-

+ 9-)

a(O,+ 4 2;)

= Y’(9-

--) 8-)

Y/(2+ -+ o+>

.- I&S. zs-

where Z,,,, and Is_ are the beam intensities for the ground state and the isomer, while Y/(9- -+ 8-) and Y’(2+ + Of) are the yields of the 9- + 8- and the 2+ -+ O+ transitions after the corrections for the feedings from the 4+ state and the internal conversion coefficients. As the experimental ratio, a value of 0.57 i 0.50 was deduced, where the large error stems from the uncertainty of 1,.,./Is-. The ratio of the cross sections was also calculated theoretically to be 0.51 by assuming the K" = 8- band as a rigid rotor with Qo = 7.3 b [ 71, the same value as the ground state. The experimental ratio is consistent with the theoretical one within the experimental accuracy. The B (E2 : 8- ---f 9-) value was extracted to be 2 f 1 e*b* from the experimental ratio. In Fig. 3, the B( E2 : 8- -+ 9- ) value is plotted, in a Qa versus B( E2) plane, together with those for a rigid rotor of K = 0 and 8. It may be

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concluded from Fig. 3 that the K” = 8- band has a collectivity as large as that of the GSB. As shown in Fig. 3, the B(E2) value of a AZ = 2 transition is much smaller than that of a AZ = 1 transition for a rotational band with K = 8. Using the B( E2) values at Qa = 7.3 b in Fig. 3, the ratio of y-ray yields, Y,( lo- + 8-)/Y,(94 S-), was estimated to be 0.16. This is consistent with the absence of a peak at 48 1 keV in Fig. 1(c) . Based on these considerations it may be concluded that K is a good quantum number, i.e. the nucleus is axially symmetric, in the K” = 8rotational band. Some K-isomer decays were recently reported to violate the K-selection rule severely in the hafnium and tungsten region [6,8]. In the case of ‘76W, the KT = 14+ isomer is reported [ 81 to decay directly to the’ K” = O+ GSB member with a branching ratio of more than 30 % . It is therefore important to measure directly the B (E2) values and Q-moments of the high-K isomer bands by further HSIB COULEX experiments. In summary, we have developed a new experimental technique by using the HSIB. The beam of ‘74Hf K” = 8- isomer was successfully applied to excite the states

Letters B 350 (1995) 169-l 72

built on the isomer by COULEX. The results suggest both a large collectivity and the axial symmetry for the K” = 8- isomer band. Though the application of the HSIB to the in-beam y-ray spectroscopy is still in its infant stage, the present data clearly show the promising future of this new technique. We thank the RIKEN accelerator crew for their effort to provide us heavy ion beams of very high intensity. We are also grateful to the staff of the JAERI tandem accelerator for providing the excellent carbonstripper foils that we used in the RIKEN RILAC injector. References [ 11 Y. Gono et al., Nucl. Phys. A 557 (1993) ~341. [2] T. Murakami et al., Z. Phys. A 345 (1993) 123. [3] A. Ferragut et al., J. Phys. Sot. Jpn 62 (1993) 3343. [4] A. Odahara et al., Z. Phys. A 350 (1994) 185. [ 51 E. Der Mateosian and L.K. Peker, Nuclear Data Sheets 66 (1992) 705. [6] N.L. Gjerup et al., Nucl. Phys. A 582 (1995) 369. [7] E. Browne, Nuclear Data Sheets 62 ( 1991) 1. [S] B. Crowell et al., Phys. Rev. L&t. 72 (1994) 1164.