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The present work reports measurements of isotope shifts and hyperfine structure in the optical spectra of two high-Z elements: Hf and U. The experiments areĀ ...
Hyperfine Interactions 74(1992)31-40

31

N U C L E A R C H A R G E RADII C H A N G E S OF U R A N I U M AND H A F N I U M I S O T O P E S D E T E R M I N E D BY LASER S P E C T R O S C O P Y

A. ANASTASSOV 1, Yu.P. GANGRSKY 1., K.P. MARINOVA 2, B.N. MARKOV 1, B.K. KUL'DJANOV 3 and S.G. ZEMLYANOI a

1Joint Institute for Nuclear Research, 141980 Dubna, Russia 2University of Sofia, Bulgaria 31nstitute for Nuclear Physics, Tashkent, Uzbekistan

The present work reports measurements of isotope shifts and hyperfine structure in the optical spectra of two high-Z elements: Hf and U. The experiments are based on the observation of the resonance fluorescence in a well collimated atomic beam, excited by a tunable dye laser. The chains of the stable Hf isotopes and of z33-z36'238U have been investigated and changes in ms nuclear charge radii have been deduced. The results have been discussed from the point of view of nuclear structure in the investigated regions.

1.

Introduction

Highly sensitive laser spectroscopy is a powerful tool for the investigation of ground and isomeric nuclear state properties through the measurement of isotopic shifts (IS) and hypcrfine structure (hfs) in atomic spectra. IS yield information about the changes in nuclear charge distribution in an isotopic chain, and the hfs allows the determination of the electromagnetic moments of nuclei. Thus, they provide an interesting and detailed insight into the nuclear collective and single particle features. In this paper, we report on the application of the method of laser-induced resonance fluorescence detection in an atomic beam for two high-Z nuclei: (a)

72Hf, with average neutron number of about 107, which is located close beyond the well-investigated rare earths in the very interesting mass region of strongly deformed nuclei. (b) 92U from the actinide region, where some peculiarities in nuclear shape, e.g. static octupole and abnormally large quadrupole deformations, have been observed.

IS measurements in the optical spectra of both elements have been performed by many authors using classical interference spectroscopy {1-4]. The A(r 2) values systematized on the basis of these experiments have fairly large errors [5]. For Hf, "Corresponding author.

9 J.C. Baltzer AG, Scientific Publishing Company

32

A. Anastassov et al., Nuclear charge radii changes of U and ltf isotopes

there are only a few investigations in which the laser spectroscopy technique has bccn applicd, e.g. [6,7]. As a rule, in these works only the relative changes in ms nuclear charge radii are deduced. It is the purpose of our work to present results of a series of high-resolution laser spectroscopic experiments. The chains of the stable Hf isotopes and of 233-236'238Uhave been investigated, so that A(r 2) values for all nuclei with the only exception of 179Hf are now known with a high degree of accuracy. Moreover, the hyperfine splitting constants of the upper and lower levels of the investigated spectral transitions and for all measured o d d - e v e n isotopes have been determined. This is our first attempt of systematic IS and hfs measurements in the two regions of high-Z, mentioned above. It is of crucial importance to have accurate experimental information on these quantities in order to test different aspects of the theoretical approaches, especially in the actinide region.

2.

Experimental

The technique used is well known: the light of a tunable cw dye laser intersects orthogonally a well collimated atomic beam. The laser-excited resonance fluorescence is detected by a photomultiplicr operated in a single photon counting mode cormected to a multichannel analyzer synchronized with the laser frequency tuning. The atomic beam is produced by evaporating the samples in a tantalum crucible with a collimator. When very high temperatures are required (T > 2000 ~ we apply laser evaporation. For this purpose, we use Q-switched N d : Y A G laser LTSPC-7 (A,= 1.06 # m , rpulse = 10 ns, maximum energy in a pulse 50 mJ, repetition rate 1 2 . 5 - 100 Hz). The laser spot size on the target can be varied with the help of a focusing system from 0.3 to 3 mm. In fig. 1 is shown a block-schematic of our experimental arrangement (described in detail in [8]). 2.1. HAFNIUM Hafnium belongs to the class of elements most difficult to vaporize. In this case, pulsed laser evaporation was found to be a convenient atomization method. Metallic hafnium samples with a natural abundance as well as up to 82% 177Hf enriched oxide samples (prepared in the form of pressed tablets) were used. The laser spot size on the target was 1 mm and the power density was -108 W/cm 2. Excitation in the atomic transition 5de6s 2 3F3-(5d26s6 p + 5d6s26p) 5G~ with = 5903/~ was used. The detection of the resonance fluorescence was achieved by observation of the spontaneous emission back to the ground state of 5d26s 33F 2 at = 5182/~, which led to a strong attenuation of the scattered laser light background. Figure 2 displays the observed spectrum: peaks corresponding to all e v e n - e v e n isotopes and to the hfs components of the odd isotopes are observed. The F W H M of the lines in the recorder spectra ( - 4 5 MHz) is determined mainly by the partial Doppler broadening, which depends on the collimation of the atomic beam and the

A. Anastassov et al., Nuclear charge radii changes of U and tlf isotopes

olomic beam

omptifier

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mullichannel analyser wilh microprocessor

scanning electronics SP- 359

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Ar*- ion loser SP-2030-20

confo\cal interferometers |

150 MHz

I wovemeter I 0.4- 4 jura

I

scanning etectronics Burl. RC-45

1

Fig. 1. Block-schematic of the laser spectrometer.

f r e q u e n c y m a r k e r s (150 MHz) 700

o co ,,-,i

~600

co b.,,.-+

500

~ 400 co 300 o 200 O tO0

0

5O

100

150 channels

200

Fig. 2. Fluorescent spectrum for the 5903/~ transition in Hf I (natural abundance).

250

300

33

A. Anastassov et al., Nuclear charge radii changes of U and Hf isotopes

34

1200

f-cO p.-

1000

~

800 ~

-Z

txl

0

OQ

7

600

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o/ o r

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I

400

200

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200

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250

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300

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350

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400

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450

channels Fig. 3. ltfs of 177H[" (/], = 5903 ,~) obtained with an enriched sample of 177HfO2. The identification of hfs components is given: F'-F denotes total angular momenta of the upper and lower levels, respectively.

velocity distribution of the atoms. Measurements with a 177Hf enriched sample (fig. 3) allowed us to identify the hfs components and to obtain the corresponding IS with higher accuracy. The results for the measured IS are summarized in table 1. Table 1

IS and A ( r z) values for Hf. A

Av 178'a MHz

A(r2) 178'a fm z

X = 5903.0 ,~

2.2.

174

1708.4 (2.6)

- 0.121 (6)

176

862.0 (1.8)

- 0,061 (3)

177

611.6 (3.4)

- 0,045 (3)

180

- 983.3 (3.2)

0.072 (4)

URANIUM

Thermal evaporation from an electrically heated crucible was used for producing the atomic bcam. A UOe(OH)3 acid solution of a few #1 was dried up on a thorium foil and thus transferred to U308. The heating of the samples to a temperature of about 1 8 0 0 - 2 0 0 0 K resulted in thermal dissociation of the oxide. Under these

A. Anastassov et al., Nuclear charge radii changes of U and 1If isotopes

Intensity, o r b . u n i t s ~ i

.

912~912

r-Hi~.lq 2

11/2..-9/2

c~

I

1~2-,,13t2

1512--15/2

t:> ,'r N

I.

_

~7/2~V//2 C: W

~D-.r7/2

Fig. 4. Fluorescence spectrum of 233U and 234U for the 5915 ,~ transition in U I.

35

36

A. Anastassov et al., Nuclear charge radii changes of U and Hf isotopes

conditions, a microgram sample provided a stable atomic beam for about 2 - 3 hours. Two types of samples were used: (a) a mixture of all e v e n - e v e n isotopes, and (b) one of the odd isotopes with the addition of 238U as a reference. Two optical transitions, beginning from the UI ground state 5f36d7s 2 5L~ were used for excitation: to the 5f36d7s7p 7L 6 l e v e l with A.= 5758/~, and to the 5 f 3 6 d 7 s 7 p 7M 7 level with ~ = 5915/~. In fig. 4, the peak o f 234U and the hfs components of 233U (,~= 5915 ,~) are shown. The FWHM of the recorder lines is 20 MHz. For the odd isotopes 233U and 235U, the hfs constants of the lower and upper levels of the investigated transitions have been determined (see ref. [10]). Here, only the experimental results on the IS are presented (table 2). Table 2

IS and A ( r 2) values for U. A

3.

Av A.238 MHz

A(r2) &23~ fm 2

~, = 5758.14 ,~

~, = 5915.42 ,~

233

- 7 7 3 0 . 5 (7.3)

- 13174.0 (3.1)

0.383 (44)

234

- 5965.6 (5.3)

- 10108.5 (4.2)

0.293 (34)

235

- 4 9 6 5 . 4 (18.8)

- 8480.5 (3.6)

0.246 (28)

236

- 2984.0 (14.0)

- 5069.5 (4.8)

0.147 (17)

A(r 2) evaluation

The difficulty of estimating the parameters necessary for the evaluation of ms nuclear radii changes A(r 2) has always been a limiting factor in the determination of nuclear properties from the optical IS. In particular, this is a serious problem for the investigated elements because of their complex spectra with very strong and practically unknown configuration interaction (CI). The problem was solved for the two elements in different ways. 3.1. HAFNIUM

According to the previous theoretical and experimental analysis [1,11], the Hf transition 5d26s 2 3F4-5d26s6 p 5G~ with X = 5453/~ can be assumed as a pure ns2-nsnp transition. Making use of this fact and on the basis of the G o u d s m i t F e r m i - S e g r 6 approximation, with experimental data from [12] and a screening ratio ),from [2], we obtained: the electronic factor E = 0.454 (27) and the specific mass shift --Vsms' A. 178 180 = ( 0 + 9 . 4 ) MHz for pure ns2-nsnp transition. Values of _/xVsms"178180 = -112 (28) MHz and E i = 0.313 (21) for the investigated transition with ~. = 5903/~ were deduced using a King plot with the data for the pure ns2-nsnp transition as a reference. In the calculation, the statistical errors of

A. Anastassov et al., Nuclear charge radii changes of U and Hf isotopes

37

all experimental IS and the theoretical errors on the parameters of the reference line are taken into account. As is seen, in the case of Hf it was possible to determine the changes in the mean-square change radii by the standard procedure described in [5]. With the above calculated parameters and a nuclear factor f ( Z ) = 39.908 GHz/fm 2 [2], we obtained the A(r 2) values given in table 1. Note that our value for A(r2) 178'18~obtained from a semi-empirical calculation and based only on optically deduced parameters coincides exactly (including the error) with the corresponding value from recent work [13], where a complex theoretical analysis of the experimental data is made. However, there is a deviation from the much larger values for A(r 2) deduced from muonic [14] as well as from electronic K X-ray spectra [15]. 3.2.

URANIUM

In this case the semi-empirical procedure, based on the G o u d s m i t - F e r m i Segr6 formula does not apply to the observed transitions because of the lack of relevant optical data for pure transitions. However, reliable information on A(r 2) can be drawn without knowledge of CI, specific mass shifts and electronic factors in the field shifts. A detailed analysis of a large number of experimental data shows that the observed IS in U are nearly proportional to the differences in neutron numbers. This indicates that the relative nuclear charge radii changes ~c~, which are equal to the relative field shift values, may be simply replaced with relative residual IS. Consequently, the absolute A(r 2} values can be derived by direct calibration, e.g. by using the X-ray data values [15] of A(r2) 233'238= 0.383 (44)fm 2. The results (see table 2) agree well with those obtained by classical interference spectroscopy [5], but have a more than twice better accuracy. However, also in this case the A(r 2) values from muonic spectra [16] are about 30% larger. 4.

Discussion

The open circles in figs. 5 and 6 summarize the results obtained, showing A(r 2) over a range of Hf and U isotopes, respectively. Since both elements belong to strongly deformed nuclei (/32 - 0.27), we shall split as a first step the variation of the ms charge radii into a volume contribution, described by the spherical droplet model [17] and a deformation tenn. This can be done by the well-known twoparameter formula A(r 2 ) =

A(r2)~+ 4-~(r2 ) ~ A(/J2),

where 13; are multiple deformations of order i.

(1)

38

A. Anastassov et al., Nuclear charge radii changes of U and Hf isotopes

72 Hf

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