Recoil Distance Transient Field g-factor measurement using EUROBALL cluster detectors

frog

Pars. Nucl.

Phys.. Vol 38. pp. 87-88. 1997 0 1997 Elsevrer Science Lrd ,n Great Bntain. All rlghrs reserved 0146-6310197 S31.00 -+ 0 00

Pnnted

Pergamon

SOl46-6410(97)00011-2

Recoil Distance Transient Field g-Factor Measurement Using EUROBALL Cluster Detectors A. JUNGCLAUS’, V. FISCHER’,

C. TEICH’, C. LINGK’,

D. SCHWALM3.

K. P. LIEBl,

J. BILLOWES2,

J. EBERTH4

and H. G. THOMAS4

‘II Physrkalisches Insritur, Unlversirat Gdttingen, D-37073 ZDepartmenr of Physics, Universiry of Manchesler. U.K. 3h4a-Planck-lnstrtut fir Kemphysik. D-69029 Heidelberg. ‘lnsrrrurfur Kernphysik. Unrwrsirur Koln. O-50937 Koln.

Abstract: Recoil

g-factors

Distance

technique possibility heavy-ion

of high-spin

Transient

Field

states (RDTF)

The measurement

is uc’ccssary

Gdftingrn,

of g-factors

to simulate

in 87Nb have been determined employing method. It has been demonstrated that

of picosecond

the complete

Germonr

Germane Germon\

in conjunction with modern highly efficient r-ray to measure in a direct way g-factors of short-lived fusion-evaporation reactions.

reactions using conventional singles severe experimental difficulties. Due Iliscrete high-spin states in this type sion of the anguiar distribution to a

D. KAST’, T. H;iRTLEIN3,

high-spin

states

detectors excited

the this

opens up a unique states populated in

populated

in (HI,zp~/n=cr)-

mode transient field (TF) techniques entails rather to the very large time spread in the population of of reaction. the assignment of the observed precesparticular state of interest is extremely difficult. It y-flux

through

the nucleus

including

all t,he trxnsi-

tion intensities. level lifetimes, sidefeeding intensities, sidefeeding lifetimes and magnet.ic moments to deduce the individual contributions of different states to the measured effect. This leads inevitably to large systematic uucertainties in the results. In order Oo avoill these problems, the RDTF method has been proposed (l,‘?), which allows to measurca t.he precession induced by the interaction of the desired single discrete high-spin state with the transient field. Fig. 1 presents a sketch target is separated from the ferromagnetic layer during the flight are Doppler shifted. When the netic layer after the flight time t = d/v, it senses Larmor-precession before it comes to rest in the

to illustrate the RDTF technique: the by a flight distance d. Y-rays emitted recoiling nucleus enters the ferromagthe transient field and experiences a non-ferromagnetic backing. By intro-

ducing a flight path the state of interest can be selected by setting a coincidence gate on the shifted component of the feeding transition ~1, thus ensuring that the level has been populated before the nucleus enters the ferromagnetic foil, and by determining the precession of the stopped component of the y-ray yS depopulating the state in this coincidence spectrum. To achieve a reasonable accuracy in the determination of the small precession angles [in the order of some ten mrad) using this coincidence technique, highly efficient y-ray detectors, which are now available in the new generation of y-ray spectrometer like EUROBALL. have to be used. We performed an RDTF experiment at the tandem accelerator of the Max-PlanckInstitut iu Heidelberg using five EUROBALL Cluster detectors and the reaction 5sNi(32S,3p)87Nb at 110 MeV beam energy to measure g-factors in s7Nb. The five Cluster detectors were positioned at O”, f55” and f125” with respect to the beam in a horizon till plane perpendicular to the magnetic field. The target consisted of a 1 mg/cm” 5sNi 87

A. Jungclaus

permanenr

magnet

N(S)

I

1

Y3 ..Q+.._@._.

g s

t .. d

I

et al

WY

0 8

target foil ferromagnetic

layer

0

non-magnetic

backin:

Fig. I: z&etch to illzlstrute the RDTF technique. foil, the stopper of a 2.8 mg/cm* Gd layer used as ferromagnetic host glued onto a thick Au backing by means of a thin (0.07 mg/cm’) I n 1a y er and cooled to liquid nitrogen temperature. A distance of 25 pm was adjusted, corresponding to a flight time of about ,i ps (v/c = 2Yo). The direction of the polarizing field of 125 mT provided by two NdFeB permanent magnets was reversed every 30 minutes. Details about the preparation of the stopper trilayer as well as the RDTF target chamber used in this experiment are given in Ref. n. l’rorn the experimental

precession

angles

-- 1 4-1

0)

n@=S(0)Jis+l ~l~terminecl

in the usual way by calculating

uf detectors

for the two field directions

N(0)

the double

counting

ratios

!Vl(-0)

of adjacent

pairs

(2)

p = iVl(O). N(4)

and t,aking into account the logarithmic slope S(0) = $ . g of the agular distribution. the g-l’actors of the 2491 keV, ‘l/2+ and the 5010 keV, 29/2- states in s7Nb have been rlcarivetl using different parametrizations of the transient magnetic field. The knowledge ol’ t,hc strcxngth of the transient field still remains the weakest point in all TF experiments. llowrvrr, since we have measured the g-factor of the relatively long-lived 21/2+ state (7 = 20(I) ps (4)) in the static hyperfine field of a Fe host for comparison (5), we have a possibility to check the TF parametrization. Assuming either a linear parametrization (6) or 111r one proposed by the Chalk River group (7) leads to g-f ac t or values which are in agreement with the static field value of g(21/2+) = 0.41(13). A full account of the data analysis aud a thorough discussion of the results will be presented in a forthcoming publication (8). (I) E. Lubkiewicz et al., Z. Phys. A335 (1990) 369 (L)) I. Birkental et al., Nucl. Phys. A555 (1993) 643 (3) (:. Teich, A. .Jungclaus, K. P. Lieb, submitted to Nucl. Instr. (4) A. .Jungclaus et al., Z. Phys. A340 (1991) 125 (.5) 1~1.We&flog et al., Nucl. Phys. A584 (1995) 133 ((i) .J. L. Eberhard et al., Ilyperfine Interactions 3 (1977) 195 (7) 0. N;iusser et al., Phys. Lett. B144 (1984) 341 (8) A. .Jungclaus et al., in preparation

Meth.