Lifetime measurements in 120Xe, using a coincidence plunger technique

Nuclear Physics A467 (1987) 528-538 North-Holland, Amsterdam

LIFETIME

MEASUREMENTS

IN “‘Xe,

PLUNGER

USING

S. HARISSOPULOS, A. DEWALD, A. GELBERG, K. LOEWENICH’, K. SCHIFFER’ and Institut

fiir Kernphysik

der Universitiit

Received

A COINCIDENCE

TECHNIQUE*

17 November

P. VON BRENTANO, K.O. ZELL

zu Kiiln, KGln, FRG 1986

Abstract: Lifetimes and side feeding times in ‘*‘Xe were measured using the recoil-distance Doppler-shift technique, taking y-singles spectra as well as yy-coincidences. The reaction 1’oCd(‘3C, 3n)“‘Xe was used at a bombarding energy of 56 MeV. Lifetimes of 1.0 (5) ps and 1.9 (6) ps respectively were obtained for the 10: and 8: states from their decay curves taken in coincidence with the strongest discrete feeding transitions. Using these lifetimes, we determined side feeding times of 2.2 (13) ps for the 10: state and 3.5 (26) ps for the 8: state, from their decay curves taken in coincidence with transitions below these levels. Decay curves taken from y-singles spectra yielded side feeding times of 2.9 (14) ps for the 10: state and 4.1 (28) ps for the 8: state.

E

“°Cd(‘3C, 3n), E = 56 MeV; measured recoil-distance Doppler NUCLEAR REACTION shift in yy-coin. ‘*‘Xe deduced T,,,, side feeding times. Enriched target Ge detectors.

1. Introduction During the last decade, the recoil-distance Doppler-shift has become a standard tool for measuring nuclear lifetimes; transition probabilities are a very sensitive test of theoretical In order to determine

the lifetime

T of a level in a y-cascade

technique ‘) (RDDS) the resulting reduced predictions. following

a compound

nucleus reaction, one has to assess the contribution of all transitions feeding this level to its recoil distance decay curve. The contribution of the observed discrete feeding from the higher-lying states in the y-cascade to the decay curve of the level considered can be derived by measuring the respective decay curves and relative y-ray intensities. However, the influence on the decay curve of the feeding from unobserved discrete or continuum y-ray transitions, otherwise known as side feeding, is hard to estimate if the feeding time T,~ associated with the unobserved transitions, in the following called “side feeding time”, is unknown. 2-6) of side feeding times have been A number of experimental investigations reported

for nuclei

of different

mass regions

produced

by various

nuclear

reactions.

l This work has been funded by the German Federal Minister for Research and Technology (BMFT) under the contract number 06OK272. ’ Present address: Istituto Nazionale di Fisica Nucleare, Sezione di Padova, Via F. Marzolo 8, I-35131 Padova, Italy. * Present address: Niels Bohr Institutet, Tandem Laboratoriet, Rise, DK-4000 Roskilde, Denmark.

0375-9474/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

S. Harissopulos

A method

used for extracting

to treat the side feeding

et al. / Lifetime

lifetimes

529

measurements

of states with considerable

time as an additional

free parameter

side feeding,

in a x2 analysis

is

of the

decay curves. However, in case the side feeding intensity l,r of a given level is considerable, normal plunger measurements show sizable errors, in particular for short-lived

states,

due to the strong

correlation

between

the estimated

the level and the respective side feeding time ‘f. ExperimentaIly, an upper limit for the side feeding time of a Ievel from its effective lifetime. The effective lifetime of a state derived distance decay curve without any feeding corrections, reflects the of the level and clearly includes its side feeding time: The longer the

lifetime

of

can be obtained from its recoilfeeding pattern effective lifetime

of a level, the slower can be its population through its side feeders. Side feeding times around 0.2 ps have been reported for the AS X0nuclei 8,9), which agree with theoretical estimations of Hellmeister et al. “) based on a statistical model calculation. However, as far as the Xe-Ba-Ce region is concerned an assumption of a short side feeding time could not be justified from our previous lifetime measurements in 12’Xe [ref. 7>], ‘24Xe [ref. ‘)] and “‘Ba [ref. “)I, carried out via the ( 13C, 3n) reaction. The situation was similar to that reported by Newton et al. “) for some heavier nuclei: Effective lifetimes of about 4ps were measured for the J” = 12+ states of the above mentioned Xe and Ba nuclei, which gave us a limit of T,~< 4 ps. The resulting B( E2) values of the lower ground state band members were strongly dependent on the side feeding time, since the estimated lifetimes of these states become shorter assuming a slow and not a prompt side feeding ‘). A correct assessment of the inffuence of the side feeding time is particularly important in those cases ‘*13) in which significant deviations from the rigid rotor values of B(E2) have been observed below backbending (prealignment anomaly). This was one of the main motivations for carrying out the present work. In this paper we report on a direct measurement of the lifetimes of the 8: and 10: levels of the “‘Xe nucleus,

using the RDDS technique,

taking not only y-singles

spectra but also yy-coincidence spectra. The side feeders of a level can be eliminated, by setting coincidence windows. Lifetimes determined from spectra gated on discrete transitions feeding the levels are independent from the side feeding time. Similar coincidence experiments on the nucleus “*Er were previously carried out by Sharpey-Shafer

et al. ‘“) and recently

by Johnson

2. Experimental

et al. I”).

method

The experiment was carried out at the FN Van de Graaff Tandem accelerator at the University of Cologne. The excited states of the 12’Xe nucleus were produced via the “‘Cd( i3C, 3nf’20Xe reaction at an incident beam energy of 56 MeV. A germanium detector at 0” relative to the beam axis was run in coincidence with three other germanium detectors at 90”. All detectors were positioned at a distance of about 6 cm from the target. Their energy resolution was 1.8-2.1 keV at 1408 keV

S. Harissopulos

530

and their

relative

efficiency

et al. / Lifetime

was about

measurements

18%. The singles

counting

rate of the 0”

detector was about 10 to 12 kHz corresponding to a coincidence counting rate of 1 kHz. Typical singles spectra are shown in fig. 1. The recoil velocity estimated from the Doppler shift was v/c = 0.85 (3)%. The target material

consisting

of a 0.5 mg/cm2

93% enriched

“‘Cd

foil was rolled

onto a 2 mg/cm2 tantalum foil. A 4 mg/cm2 gold foil was used as a stopper for the recoils and a 50 mg/cm’ bismuth foil rolled onto a 100 mg/cm2 Cu foil was used to stop the beam. The roughness of both the stopper and target foils checked by a microscope was smaller than 2 pm. The plunger apparatus employed for the RDDS technique, is described in ref. 16). Relative distances could be directly read out by a magnetic transducer with an accuracy of 0.2 Frn in the O-40 km range. The capacity method 17) was used to (a) determine the absolute distances by measuring the capacity C between target and stopper foils at given adjustments d between them and further extrapolating the l/C versus d curve to l/C = 0 and (b) check the stability of the distances during the whole duration of the experiment. Coincidence spectra were taken only at four distances (7.6, 10.1, 13.9 and 18.4 Fm), each distance requiring a 24 hour run, *lo3

Q-

a+-, 6+ (u)

d = 7.6

pm

4 10 m *

Z6 0

0

5 12

10 B

5 2100

2200

2300

2400

Channel Number Fig. 1. Doppler-shifted

(s) and unshifted (u) peaks in the y-singles distances d (0" detector).

spectra

of “‘Xe

at three

recoil

531

S. Harissopulos et al. / Lifetime measurements

whereas

y-singles

spectra

were taken

and 200 pm. (2 hours each.)

at four additional

of the experiment and not switched off. The thermal to be less than 0.4 pm during a 24 hour run. Since coincidence and stopper, side feeding strongest.

spectra

distances:

23.6, 28.6, 33.2

The 13C beam was kept stable for the whole duration

could

be taken

drift of the foils were found

only at four distances

between

target

it was necessary to estimate the range in which the influence of the on the decay curves of the 8: and 10: states is expected to be the

Fig. 2 shows calculated decay curves for different side feeding times of a state with a lifetime of 1 ps and a side feeding intensity of 50%, fed through the decay of a level with an effective lifetime of 2 ps. This is what one could expect for the 10: state I’).

G

2

PSI

/Sf-__.

a: 1.

0. 0-

?3f

\\\

0. 6-

I \

D=r--

a: b: c: d:

covered

\n

0 1 3 5

ps ps ps ps

0. 4-

0. 2-

0

0

5

lb

15

Distance

20

25

cl

[pm1

Fig. 2. Calculated decay curves for different side feeding times rSr = O-5 ps of a level with a lifetime of 1 ps and 50% side feeding intensity, which is fed from a level with an effective lifetime of 2 ps. The vertical lines show the range of distances where coincidence spectra were taken.

The figure suggests that the slower the side feeding, the longer are the distances where spectra have to be taken in order to extract the influence of the side feeding on the decay curve. Since the expected side feeding time was about l-5 ps, spectra at distances in the O-20 brn region should be taken.

3. Data analysis and results The coincidence data for each distance matrices stored on disc.

were sorted

off-line

onto three 4K x 4K

S. ~ar~sso~ul~~ et al. / ~~~ei~~e ~gusur~~~nts

532

Energy gates were set on the 90’ detectors where no Doppler-shifted peaks occur. Since the 90” detectors covered a large solid angle (Ad - 0,3rr), which gave rise to a broadening of the peaks, rather wide energy windows of 16 keV width for the 2:-+0: transition and 21 keV for the 12: + 10: transition were set in order to be sure that the detection efficiency of y-rays emitted in flight does not vary with the distance between target and stopper. The coincident background was subtracted from the peak-gated spectra in a standard way. A coincidence spectrum at 90”, with completely open energy gate is shown in fig. 3.

t, 400

600

800 1000 Channel Number

1200

Fig. 3. 90” coincidence spectrum of ‘*‘Xe with completely open energy gate at a recoil

distanceof 7.6 pm.

The intensities I, and 1, respectively of the unshifted and Doppler-shifted components of a line by which a level decays, were determined from the gated 0” spectra as well as from the y-singles spectra. The sum of the Doppler-shjfted and unshifted transition in the coincidence 0” spectra taken at different peaks of the 2f+0: distances with completely open energy gate was used to normalize the 0” spectra gated with the transitions of interest, whereas the y-singles spectra were normalized to the 279 keV Coulomb excitation line of gold (stopper foil). Some typical gated 0” speetra are shown in fig. 4. The decay of the levels of interest was analysed using the program FXTRDDS I’). The lifetime T of a given level was determined by fitting its decay curve with the following function: R(d=uE)=C~~(~~e-“i~-~e-f”)/(~i--)], i

where R(d) is the intensity of the decays at the distance d between is the *‘effective” lifetime of the intensity A. The effective lifetime

unshifted component of a line by which the level target and stopper, z is the time of flight and Fi ith state feeding the level considered, with an of a level is the time required for the unshifted

S. ~~risso~~l~s

t

25

1000

d = 7.6

pm

! 1060 Channel

Fig. 4. Doppler-shifted

fraction

R(d)

(s) and unshifted 2:+ 0: transition,

to decrease

to l/e.

533

meusurements

et aL f Lifetime

r 1120

I 1 if80

Number

(u) peaks in the coincidence spectra of “OXe, gated with the at three recoil distances. (0” detector.)

The intensities

J were taken

distribution measurement performed by Loewenich et al. *‘). The sum I,, + 1, of each transition considered was not statistically distances where spectra were taken. Hence, an additional point

from the angular different at the at an absolute

distance d = 0 p.m between target and stopper, where no DoppIer-shifted peak occurs, was taken into account by fitting the data. This point is given by the average of the sums I, + I, determined at distances where spectra were taken. The decay curve of a level, obtained from spectra taken in coincidence with the strongest discrete transition feeding this level is not influenced by the side feeding of the level. Lifetimes of 1.0 (5) ps for the 10: state and 1.9 (6) ps for the 8: state respectively were determined, by fitting their decay curves taken in coincidence with the 12:+ 10: and 10:-+8: transitions. These values are independent of the side feeding time. The fit curves are shown in figs. 5 and 6 and labeled with the letter a. Curves labeled b represent expected coincidence rates calculated with the lifetimes obtained from the y-singles spectra assuming a side feeding time of ~~~= 0. If this assumption were correct, curves a and b would be identical. However, it is obvious that the experimental data (solid circles) are not consistent with curves b. Both

S. Harissopulos

534

z a

et al. / Lifetime

measurements

2500 10+

+

8+

2000-

1500-

IOOO-

500-

0

1 0

5

IO

15

20

25

Distance

d

[pm1

Fig. 5. Decay of the 10: state of “‘Xe Intensity (in arb. units) of the unshifted peak (solid circles) obtained from spectra gated with the 12: + 10: transition, versus recoil distances. Curve a is the fitted decay curve from which the lifetime of the state was derived. Curve b is the decay curve for the lifetime obtained from the y-singles spectra assuming rSr = 0. The error in the zero of the distance scale is indicated by the horizontal error bar. This applies also to the next three figures.

5 Distance

d

[pm1

Fig. 6. Decay of the 8: state of ‘*axe. Intensity (in arb. units) of the unshifted peak (solid circles) obtained from spectra gated with the 10,+ + 8: transition, versus recoil distances. Curve a is the fitted decay curve from which the lifetime of the state was derived. Curve b is the decay curve for the lifetime obtained from the y-singles spectra assuming rSr = 0.

figures show that the range of distances where spectra were taken was chosen correctly since the contribution of the side feeding on the decay curves described by the difference between curves a and b, could hardly be observed outside the interval 5-25 km. Assuming that the lifetimes given above are correct, an effective side feeding time for the 10: and 8: states was determined by fitting decay curves obtained (a) from the y-singles spectra and (b) from the sum of the spectra taken in coincidence with transitions below the levels considered. In both cases the effective side feeding time

S. ~~~~sso~ulos e8al. / L$eferime me~suremenis was

treated as a free parameter.

From the y-singles

535

spectra

we obtained

effective

side feeding times of 2.9 (14) ps and 4.1 (28) ps respectively for the 30: and 8: states. From the coincidence data effective side feeding times of 2.2 (13) and 3.5 (26) ps respectively were determined for the 10: and 8: states. Figs. 7 and 8 respectively show fit curves from which side feeding times were obtained for the 10: and 8: levels. In both figures, curves labeled (a) concern fits of the decay curves taken from y-singles spectra (circles), whereas curves (b) concern

fits of decay curves obtained from the sum of the spectra taken in coincidence with alf transitions below the level considered (stars). A check of the analysis of the coincidence data is the comparison of decay curves (a) and (b). These decay curves are expected to be identicaf. As can be seen from

“;; -

cc

lOOO-

lo+

+

et

BOO600--

400 ;i,

2u~~ 0

5

10

15

20

25

30

:

Distance d [pm1 Fig. 7. Intensity of the unshifted peak (in arb. units) showing the decay of the 10: state in **‘Xe. (a) Decay curve taken from y-singles spectra (circles). (b) Decay curve taken in coincidence with all transitions below the 8: level (stars). The two curves have been normalized so as to coincide at d = 0.

Distance

d

[pm

Fig. 8. Intensity of the unshifted peak (in arb. units) showing the decay of the 8: state in ‘*@Xe.(a) Decay curve taken from y-singles spectra fcircles). (b) Decay curve taken in coincidence with all transitions befow the 6: tevel {stars). The two curves have been normalized so as to coincide at d = 0.

er al. ,! Lifetime measurements

S. ~arissu~~las

536

figs. 7 and 8 the differences

are not statistically

and side feeding times reported of error analysis,

significant.

The errors in the lifetimes

in this work, were obtained

using standard

methods

4. Conclusions Relative

B(E2)

values

of the ST, 10: and 12: states in ‘**Xe determined

in this

measurement, are shown in fig. 9. They show deviations from the predictions of both the Rotor and IBA-2 [ref. *‘)I models. Fig. 9 also indicates that the deviation from the theoretical value of B(E2; ST+ 6:)/B(E2; 2:+ 0:) and B(E2; IO?-+ 8:)/B(E2; 2:3 0:) of the 8: and 10: states is indeed a real effect, since the influence of the side feeding time has been taken into account in this work. The relative B( E2) value of the 12: state is only a lower limit. For the 12: state only an effective lifetime could be measured in this experiment.

I . Exp. mlm

oc

2

4

6

8

IO

12 Spin

Fig. 9. Relative

The side feeding

B(E2) values

for “‘Xe.

times reported

Values for I = 2,4,6

in this paper

I

are taken from Loewenich

I*).

for the 10: and 8: states of “‘Xe,

are a clear evidence of the existence of a slow and not a prompt side feeding by carbon induced reactions. Therefore an accurate knowledge of the side feeding times of excited nuciear states is necessary in arder to get correct lifetimes. Assuming that the side feeding of a level consists of a high number of small contributions of different decay paths of similar types, then one might expect that the side feeding time of a given state and the feeding time associated with observed discrete y-ray transitions feeding the state considered lie very closely. A comparison of the side feeding times measured for the 10: and 8: states in ‘*‘Xe with the determined effective lifetimes of the respective levels feeding them (see table I), supports such an assumption. However, more accurate information about the side feeding time of excited states of other nuclei has to be obtained in order to test such an assumption.

S. Harissopulos et al. / Lifefime measurements

537

TABLE 1 Lifetimes 7, effective lifetimes ? and side feeding times rsr determined in this work. The relative side feeding intensity I,, is also given Feeding 10:

Level 8+I (ps)

I,f (%)

Tsr (ps)

7 (ps)

7 (PS)

1.9 (6) “)

22.8 d,

3.5 (26) “) 4.1 (28) ‘)

3.8 (2) “) 3.9 (4) b)

5.6 (10) “)

7

Feeding level 12:

Level 10: (ps)

I,, (%)

T,i (ps)

7 (ps)

1.0 (5) “)

48.4 d,

2.2 (13) l’) 2.9 (14) b,

2.4 (5) “) 1.8 (8) b,

7

“) b, ‘) d,

The authors

levels 9

Value Value Value Value

would

obtained from the coincidence data. obtained from the singles data. taken from Loewenich ‘s). taken from Loewenich 20).

like to thank

W. Krips for his help in data processing.

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measurements

14) D. Ward, H.R. Andrews, J.S. Geiger, R.L. Graham and J.F. Sharpey-Schafer, Phys. Rev. Lett. 30 (1973) 493 15) N.R. Johnson, Proc. Symp. on Electromagnetic properties of high spin states, Stockholm, 1985 16) H. Hanenwinkel, Diplomarbeit, Kiiln 1981, unpublished 17) T.K. Alexander and A. Bell, Nucl. Instr. Meth. 81 (1970) 22 18) K. Loewenich, Diplomarbeit, Kiiln 1984, unpublished 19) N. Schmal, Program FITRDDS, unpublished 20) K. Loewnich, K.O. Zell, A. Dewald, W. Gast, A. Gelberg, W. Liebea, P. van Brentano, and P. van Isacker, Nucl. Phys. A460 (1986) 361 21) H. Hatter, Diplomarbeit, Koln 1984, unpublished