Gamma-ray doppler shift measurements in 93Mo

I!-! l.E.l: 2.B

Nuclear

Physics A238

Not to be reproduced

GAMMA-BY

by photoprint

DOPPLER

A. CHARVET, Institut

(1975)333-339;

@

forth-Holland

P~bi~shiff~ Cu., Anlsierdam

or microfilm without written permission from the publisher

SHIFT MEASURE~NTS

R. CHERY, R. DUFFAIT

IN 93Mo

and M. MORGUE

de Physique Nucle’aire, Universit6 Claude Bernard Lyon-I et IN2P3 43, Bd du II noaembre 1918, 69621 Villeurbanne, France

Received 27 June 1974 (Revised 11 September 1974) Abstract: The lifetimes of a number of levels in 93Mo have been measured with the Doppler shift attenuation method (DSAM) using the g3Nb(p, ny)93Mo reaction and observing the y-rays emitted by the “MO nuclei recoiling in the self-supporting 93Nb target. The lifetimes are: 115:64(: fs, 1362.95 keV; 340~:~~ fs, 1477.15 keV; 38_,, + *’ fs, 1492.30 fs; > 280 fs, 1520.28 keV; 9.5~~~ fs, 1694.5OkeV;

205_,, f loo fs, 2141.10 keV;

> 390 fs, 2246.80 keV;

2303.90 keV; > 500 fs, 2355.25 keV; > 900 fs, 2409.00 keV; 160+$

> 290 fs,

fs, 2430.90 keV; > 440

‘60 fs, 2535.20 keV. The results are compared fs, 2440.20 keV; 87235: fs, 2479.00 keV; 130_,, to previous experimental or theoretical investigations. E

1. Introduction

The levels of 93Mo have been investigated by several authors in recent years, due to the proximity of 92Mo which has a closed shell of neutrons. In particular, many spin and parity assignments have been made using various reactions I- I’). In addition, this nucfeus has been investigated through the radioactive decays of 93mNb, 93gNb and 93mM~ [ref. ‘“)I and a few multipolarities have been determined. Moreover, calculations have been performed 13#14) which show that some of the excited states of 93Mo may be described in terms of a coupling between the single neutron and the excited core of “MO. Nevertheless, this model has to be checked with transition probabilities. unfortunately, onIy the lifetime of the 2161.90 keV level 12) can be measured by means of the delayed coincidence method (T+ = 5.5 ps). It is the aim of the present work to measure lifetimes of levels in 93Mo by means of the Doppler shift attenuation method (DSAM). 2. Ex~er~ental

methods and results

The levels of 93Mo were popuIated using the 93Nb(p, ny)93Mo reaction. From the analysis of the 1477 keV y-ray intensity at various energies, the Q-value was deter-, mined to be 1190+5 keV. The proton beam was supplied by the Lyon University 333

334

A. CHARVET

et al.

Van de Graaff accelerator at energies ranging from 2.9 to 4.0 MeV. The target consisted of a natural niobium foil which was approximatively 10 mg/cm* thick. The y-rays from the reaction were detected in a 60 cm3 coaxial Ge(Li) detector which had a resolution (FWHM) of 3.0 keV at 1332 keV. 2.1. THE LEVEL SCHEME

As mentioned in sect. 1, a number of levels of 93Mo have previously been determined. The spectrum for 8, = 90” shown in fig. 1 was recorded at Ep = 4.00 MeV. Many observed lines are due to parasitic reactions on duraluminium which is present in the reaction chamber. Besides, a few y-rays can be ascribed to the g3Nb(p, 7) and the g3Nb(p, p’) reactions. The proposed level scheme is given in fig. 2. The most probable spins and parities were deduced from previous studies ’ -r2). Most of the y-rays decayed to the ground state and only four cascades were observed. These cascades were identified by determining the energy of the reaction threshold of each y-ray in the spectrum. Good agreement was obtained with other investigations using the 93Nb(p, n)93Mo reaction r*“). However, the 944 keV level [which was found r*“) to be weakly excited through the g3Nb(p, n) reaction] and the 2182 keV level were not observed. In addition, the 2.44 MeV level which has previously been found to be composite was separated into levefs at 2430.9 keV and 2440.2 keV. According to refs. 4, 6P**9), they were assigned to the states (3’) and @*) respectively.

Fig. I. Gamma spectrum between 600 and 2600 keV obtained with 4 MeV protons for 8, = 90”. Only the y-rays ascribed to the g3Nb(p, ny)93Mo reaction are mentioned.

-Nb(p,

ny)=Mo

335

7h

Fig. 2. (a) Level scheme of 93Mo obtained from radloactive decays of 93m*eNband y3mM~ [ref. 12)]. (b) Level scheme of 93Mo obtained from the present work for E,, = 4 MeV. The more probable spins and parities are indicated according to previous investigations l-12). 2.2. DOPPLER

SHIFT

MEASUREMENTS

Up to now, a few DSAM measurements have been performed in nuclei with mass number A around 90 [refs. 15S16,21)]. In the particular case of the 93Nb(p, ny)93Mo reaction, the spectra were measured at a beam energy of 100-300 keV above the threshold for each level. In that case, the initial mean velocity of the recoiling 93Mo nucleus is practically insensitive to the angular distribution of the outgoing neutron [ref. r6) J. The recoil energy of the 93MO ion IS about 43 keV for 4 MeV protons. The y-spectra were accumulated at eY = 46” and 8, = 132.5”. The gain of the system was continuously monitored with radioactive sources (60Co, ’ 6Co or 226Ra depending on the level) which were also used to determine the magnitude of any slight gain shift

“)

@+I

2535.20

0.29 +0.06 0.21 f0.04

1.01~0.20 0.75kO.15

E2

2535.20

“) Probable assignments deduced from previous studies i--12I. “) Ref. 21). ‘) Ref. f 2).

($9 3-j

2479.00

(El)

< 0.062

< 0.08

(%+)

2440.20

2479.00

0.17 io.04

0.55&0.13

E2

(it”)

2430.90 Ml(+E2)

2430.90

G+)

2409.00

963.07

< 0.034

< 0.11

E2

13o+60 -35

-30

87+50

160*90 -5s > 440

> 900

> 500

< 0.055

< 0.17

(El)

2355.25

2409.00

2355.25

&,

> 390 > 290

< 0.07

-65

205+1oo

< 0.09

(%&+I

2246.80

0.140~0.035

0.41*0.10

E2

E2

684.75

2141.10 < 0.07

q+

2161.90

-25

95+5o

> 280

-13

< 0.10

(*+I

2141.10

Ml

-130

3Sfz0

El

(;+,

1694.50

< 0.085 0.26 rtO.05

< 0.15

0.53~0.10

Ml(+E2)

1520.28

1694.50

0.48 *0.08

0.87rtO.15

Ml

340+1ao

-45

115+eo

Ml(SE2)

‘+

1520.28

1492.30

0.082~0.030

0.14+0.05

0.22 10.05

E2

0.35hO.08

-- ---

826.14

$+

1492.30

1477.15

Ml(+E2)

Mult. “)

769.65

#+

1477.15

1362.95

(keV)

Transition

TABLE 1

b,

79200ft1500c)

440+23o b) -130

60140

-60

17o+*o tr)

(fs) _-

Other measurements

-0.040

114+0.070

x 10-h . -0.7

2 4+0.9

(4.0 ‘-;:g,

2 4+1.2 * -0.9 < 0.09

< 0.47

< 7.4 >: 10-J

< 2.9~ lO-3

< 0.2

2.710.4

. -1.1

3 4fl.6

0.075&0.025

< 0.04

-4.5 ,~275+0.'45 . -0.095

.

13.5+*.5

0

reaction

jMl2 (WA’.)

factors for radiative transitions in g3Mo excited by the g3Nb(p, n)‘jg3Mo up to 4 MeV

2303.90

H’

1362.95

_.---.

IZ

Level

(keV)

Mean shifts, attenuation factors, mean lifetimes and Weisskopf

2 %

$

2

?

iz

93Nb(p,

337

ny)93Mo

which might have occurred. The counting rate in the Ge(Li) detector was kept constant during the accumulation. The Doppler shifts presented in table 1 result from the average of at least three spectra at each angle in order to ensure the reljability of the data. The centroids of the peaks were determined from the analysis of spectra with the help of the SAMPO program I’). In this program, the peaks are fitted with a Gaussian having exponential tails. The background (which is approximatively the same for forward and backward angles) is represented by a parabolic line. The Doppler shift AE, is related to the attenuation factor F(z) by d E, = Ey 5 (cos 8,, -cos

e,,) F(z),

where u is the initial velocity of the recoiling nucleus. As an example, the experimental Doppler shifts for the decay radiation from levels at 2479 and 2535 keV are shown in fig. 3.

I

1

3300

.:L’50

Fig. 3. Observed

2.3.

DETERMINATION

OF

Doppler

I

!

i

3350

3xG

3450

shifts

J f~rmiw:

for the lines of 2479.0 and 2535.2

keV.

LIFETIMES

The measured F(z) have to be related to the lifetimes of the levels. Since there are no experimental measurements of the energy loss of 93Mo in 93Nb, the energy loss was estimated from the theory of Lindhard, Scharff and Schiott la) supplemented by the work of Blaugrund l “). Though the electronic and nuclear energy loss rates are not weli known in medium weight nuclei, it has been verified “) that the calculations of F(z) according to Blaugrund lead to lifetimes which are in good agreement with those given by other

338

A. CHARVET et al.

methods for nuclei ranging from 44Ca to ‘36Ba. Hence, an uncertainty for the attenuation curve was obtained by assigning a 20 % uncertainty to the total stopping power. An example of a calculated F(z) versus r curve is shown in fig. 4. The experimental data and the lifetime values are given in table 1. The lifetimes of the 1362 keV

Fig. 4. Attenuation mental attenuations

curve for the 1363 and 1477 keV states. The dashed lines indicate the experiand the corresponding mean lifetimes. Curves II and III correspond to a 20 ‘A decrease or increase in the stopping power.

and 1492 keV levels are in reasonable agreement with recent measurements performed with the help of the “Mo(‘~C, “C) reaction “I). Nevertheless, there is a large discrepancy for the 1694 keV level, which cannot be explained yet. 3, Discu~~o~ The Weisskopf factors determined from the present study are given in the last column of table 1. The particle states of “3Mo are expected at 1363 keV (g+), 1492 keV (d,) and 2303 keV (h,) (th e s+ state at 944 keV was not observed in the present study). As a matter of fact, those transitions show Ml or El hindrance factors which are in agreement with single-particle excitations. The states at 1477 keV (s’), 2162 keV (q--‘) and 2425 keV (%j-‘) result from the coupling 13,14) of the vd, state to the proton pair excitations (rcg$in g2Mo situated at 1.54 MeV (2+), 2.33 MeV (4+) and 2.79 MeV (8+). However, the 684 keV transition which connects two of these states has a rather small enhancement factor (/Ml2 = 2.7 W.U.) whereas that of the 1477 keV transition is more important (I&q2 = 13 W.U.). Several other even-parity states can be also ascribed to the (zg9)“(vd+) coupling: 1520 keV (s’), 1694 keV (s’), 2247 keV (q+), 2409 keV (8’) [refs. 13#‘“)I or to other couplings such as (rcg+)‘(ts,). The Weisskopf factors for the E2 and Ml transitions de-exciting those levels are generally in the range expected for single-particIe states with the exception of the 2409 keV transition which appears to be more hindered.

=Nb(p,

II#‘~Mo

339

With regard to the odd-parity states, they are likely to result from couplings of the neutron excitations to the 5- state situated at 2526 keV in “MO. It should be noted that the Weisskopf factor of the 2355 keV El transition is rather small. Finally, it appears that a more detailed unified model calculation including most of the possible configurations is now required to make full use of the experimental results. The authors wish to thank Dr. C. MiehC and Dr. G. Walter of the CRN Strasbourg for their assistance in calculating the attenuation functions.

of

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21)

E. Finckh and U. Jahnke, Nucl. Phys. All1 (1968) 338 H. J. Kim, R. L. Robinson, C. H. Johnson and S. Raman. Nucl. Phys. Al42 (1970) 35 C. M. Fou, Nucl. Phys. Al92 (1972) 145 M. A. Moinester, G. Finkel, J. A. Alster and P. Martin, Nucl. Phys. A202 (1973) 473 D. L. Matthews, F. F. Hopkins, P. Richard, G. W. Phillips and C. F. Moore, Phys. Rev. C5 (1972) 1390 J. B. Moorhead and R. A. Moyer, Phys. Rev. 184 (1969) 1205 R. C. Diehl, B. L. Cohen, R. A. Moyer and L. H. Goldman, Phys. Rev. Cl (1970) 2132 S. A. Hjorth and B. L. Cohen, Phys. Rev. 135 (1964) B920 H. Ohnuma and J. L. Yntema, Phys. Rev. 178 (1969) 1855 J. B. Ball, Phys. Lett. 41B (1972) 55 W. Dtinnweber, C. Borcea, P. von Brentano and E. Grosse, 2. Phys. 254 (1972) 133 A. Charvet, R. Chery, Do Huu Phuoc, R. Duffait and M. Morgue, J. de Phys. 35 (1974) 199, and references therein J. Vervier, Nucl. Phys. 75 (1966) 17 K. H. Bhatt and J. B. Ball, Nucl. Phys. 63 (1965) 286 W. Beens, Ph.D. thesis, Free University of Amsterdam (1973) R. D. Gill, J. M. G. Caraca, A. J. Cox and H. J. Rose. Nucl. Phys. Al87 (1972) 369 J. T. Routti, CERN report no. 3414 (1970) J. Lindhard, M. Scharff and H. E. Schiott, Mat. Fys. Medd. Dan. Vid. Selsk. 33 (1963) no. 14 A. E. Blaugrund, Nucl. Phys. 88 (1966) 601 T. R. Fisher and P. D. Bond, Part. and Nucl. 6 (1973) 119 G. A. Gill, R. D. Gill and G. A. Jones, Nucl. Phys. A224 (1974) 152