The attenuation of the angular correlation of 181Ta 133–482 keV cascade in liquids

Physica 28 705-710

Boiek, E. Hrynkiewicz, A. 2. Konieczny, Z. Ogaza, S. Rybicka, M. Szymczyk, S. 1962

THE ATTENUATION OF THE ANGULAR CORRELATION OF 18lTa 133-482 keV CASCADE IN LIQUIDS by E. BOZEK, A. 2. HRYNKIEWICZ, 2. KONIECZNY, M. RYBICKA and S. SZYMCZYK

S. OGAZA,

Instytut Fizyki Jadrowej, Krak6w Instytut Fizyki UniwersytetuJagiellodskiego,Krakow

synopsis Measurements of the time dependence of the angular correlation anisotropy for 133-482 keV 7-y cascade in 1slTa were carried out. An exponential attenuation of the anisotropy was found in the case of Ta ions in concentrated sulphuric acid. The experimental values of the attenuation constant 12 for different temperatures are compared with the theory.

Introduction. The measurements of the attenuation of the angular correlation of y rays emitted in cascade provide a means of studying the interaction of the nucleus in the intermediate excited state with extranuclear fields and give some valuable information on the structure and the microdynamics of substances used as a radioactive source. Besides , a knowledge of the character and the time dependence of the perturbation of the angular correlation plays an important role in the interpretation of the results of magnetic moment measurements for the excited nuclear states when the integral method of the angular correlation in the magnetic field is used. Only in the case of an unperturbed angular correlation is the interpretation of the experimental results straightforward. When the correlation is attenuated the attenuation coefficients and their time dependence must be determined to avoid serious systematic errors in the evaluation of the magnetic moment. In the case of diamagnetic radioactive ions in liquids a perturbation of the angular correlation is only caused by quadrupole interactions. In liquids of a small viscosity when the correlation time TV,describing the rotation of molecules, is much shorter than the mean life time T of the excited nuclear state, the directions of the electric field gradient average out and the quadrupole interaction does not effect the angular correlation of y rays. When the condition TV< T is not fulfilled the angular correlation anisotropy is time dependent and this dependence has an exponential character. The attenuation coefficients Gak(t) in the angular correlation formula

w(e) = 1 + 5 &a(t) A2#2&os k=l

-

705 -

0)

(1)

706

BOZEK,

HRYNKIEWICZ,

KONIECZNY,

OGAZA,

RYBICKA,

SZYMCZYK

take the formr) (2) where Azfi = &

(+y(g>s%

2k(2K + 1)[41(1+ P(21

1) -

2k(2k + 1) 1)2

11

(3)

eQ and l are the quadrupole moment and the spin of the nucleus in the excited state,
a3 kT

rJ

(4

where a is the diameter of a molecule, 7 is the viscosity coefficient and T is the absolute temperature. In the case of paramagnetic radioactive ions in liquids the attenuation of the angular correlation anisotropy can also be due to the magnetic hyperfine structure couplinga) between the nucleus and the electronic structure of the ion. This effect depends on the hfs coupling constant and is governed by the electron spin relaxation time in a similar way as the quadrupole interaction effect is governed by the correlation times). Recent developments of the fast coincidence techniques make it possible to investigate the time dependence of the angular correlation anisotropy in the nanosecond range. The cascade 133-482 keV in 1slTa is very suitable for this purpose. The half-life of the parent larHf isotope is 46 days. The mean life time of the intermediate 482 keV state is 15.6 nsec, the anisotropy of the angular correlation is about -40 percent and the first 134 keV photon of the cascade is emitted from the metastable state with the mean life time 2.6 x 1OW set which rules out the possibility of a perturbation of the angular correlation due to the rearrangement of Ta electron shells after the /P decay of rsrHf. Early measurements of the angular correlation attenuation in liquid sources as a function of viscosity were performed by He nni g and St e f f e n4) and by the Zurich groups) for the 122 nsec state of 247 keV energy in 1riCd. The source used was in the form of InCls solution in water-glycerine mixtures. The viscosity was changed either by changing the glycerine concentration4)5) or by changing the temperatures). These measurements were carried out by the integral method with a coincidence circuit of which the resolving time was longer then the lifetime of the investigated state. In 1954 the results of the time spectra measurements with the delayed coincidence technique were reported by Fraser and Miltons) and by the Zurich group7). The agreement of their results with the theoretically predicted exponential attenuation of the anisotropy was satisfactory.

ATTENUATION

The integral 133-482

keV

OFTHE

measurements Ta

cascade

OF A 1slTa CASCADE

ANGULARCORRELATION

of the attenuation

in liquid

sources

of the anisotropy

of different

viscosities

707

for the were

performed by Paul and Steffens). Azuma and Lewiss), with the delayed coincidence technique, showed that, for the source prepared by fusing HfOs with KHF and dissolving the product in water, the Ta cascade anisotropy is reduced to one half in about 54 nsec. In the present paper we give the results of the measurements

of the time

dependence of the angular correlation anisotropy for an Hf source in concentrated sulphuric acid at different temperatures. Experimental. The source was prepared from the HfOs powder irradiated in the Warsaw reactor by dissolving in hot concentrated hydrofluoric acid, precipitation of Hf (OH) 4 and dissolving this in concentrated sulphuric acid. The viscosity of the sample was changed by changing the temperature of the water or of the oil bath. The bath temperature was stabilized with a thermostat. The y rays were detected with scintillation counters with RCA 6810 A photomultipliers and NaI(T1) crystals 3 mm thick and 14” thick for 133 keV and 482 keV lines respectively. Suitable Cu, Cd and Pb (for the 482 keV counter) absorbers were used for cutting down the intensities of the lower energy lines. The time spectra of fast coincidences for the 133-482 keV cascade were taken with a conventional fast-slow coincidence circuit with a time to pulse hight converter and a multichannel analyzer. Time spectra for 90” and 180” counter settings were registered in two parts of the analyzer memory. During the run the output pulses from the single channel analyzers set on 133 and 482 keV lines in the slow coincidence branches and the pulses from the slow coincidence circuit were monitored separately with two sets of scalers for both counter positions. The change of the position of the moving counter and the switching of the multichannel analyzer and scalers were performed automatically *). The resolving time 270 of the fast coincidence circuit was determined with prompt coincidences of the annihilation radiation from a ‘JsNa source. For energies 5 11-511 keV and 133-511 keV the values of 3.2 nsec and 5.7 nsec were obtained

respectively.

Results and discussion. In the first part of the experiment the mean life time of the 482 keV Ta level and the time dependence of the angular correlation anisotropy for HfF4 in concentrated hydrofluoric acid were measured. The obtained half life T+ = 10.8 nsec is in a good agreement with that given by Nainania). It was found that the angular correlation of the 133-482 keV cascade is unperturbed in HF. The experimental anisotropy *) A detailed description oftheapparatus will

be

published in Nukleonika.

708

BOiEK,

HRYNKIEWICZ,

KONIECZNY,

OGAZA,

RYBICKA,

SZYMCZYK

A = -30.3 f 0.9 percent is constant in time (Fig. 1) and, corrected the counter geometry, corresponds to the maximum values obtained

for by

other authorsrr) 12). I

,L,._, 2

4

6

8 I

IO I

12

14

___L~~,

r.5

Fig.

1. Time for

dependence

133-482 keV

Fig. 2. Time

of

cascade

dependence

for 133-482 keV cascade

I8 1

2% , 22 1

24 I

26 1

28 I

30

I nsec

-

Time

Delay

the

angular

correlation

of 1slTa in concentrated

of the angular correlation of 1slTa in concentrated

anisotropy HF.

anisotropy H&04.

The results of the time dependence measurements of the angular correlattion anisotropy for the same cascade in concentrated H&04 for four temperatures 1O”C, 3O”C, 60°C and 80°C are shown in fig. 2. The experimental points were calculated from the time spectra of fast coincidences obtained for both counter settings after corrections for chance coincidences. The

ATTENUATION

OFTHE

OF A rsrTa CASCADE

ANGULARCORRELATION

709

straight lines representing the attenuation of the anisotropy are determined by the least square method. For all four temperatures time dependence of the anisotropy is exponential in agreement with the Abragam and Pound theory. The extrapolation of the attenuation curves to zero delay time gives the anisotropy value of -3 1.5 percent, in excellent agreement with the result obtained for the HF sample. From the known shape of the angular correlation function for the investigated cascaders) it is easy to deduce that the influence of the G&) Aa term on the time dependence of the anisotropy is quite negligible for t > 0.3 nsec and the attenuation is then entirely determined by the behaviour of Gs(t) coefficient. The numerical values of the attenuation constants are given in table I. TABLE T in OK hs in 107 set-1

283

1

14.87 + 0.98

I

303

1

11.10 & 0.30

333

1

7.87 T 0.46

353

)

6.68 f

0.23

According to formula (3) with TVvalue given by (4) and with the assumption that the quadrupole interaction is independent of the temperature, the temperature dependence of 1s should be the same as that of q/T.The viscosity coefficient of the sulphuric acid used for the source preparation was measured as a function of temperature with a Hoeppler viscosimeter.

Fig. 3. Comparison

of the experimental of 1s. Both

and the theoretical

curves normalized

temperature

dependence

at 30°C

In fig. 3 the experimentally obtained temperature dependence of ils is compared with the theoretical temperature dependence, assumning that

7 10

ATTENUATION

OF THE

ANGULAR

CORRELATION

OF A

18lTa

CASCADE

the latter is determined by q/T. For the sake of comparison both curves are normalized at 30°C. With increasing temperature the experimental ii2 decreases much slower then the theoretical one. The introduction of a microviscosity coefficient instead of q does not improve the situation because its temperature dependence should remain the same. It is quite possible that the size of a complex surrounding the tantalum ion changes with temperature which can change the temperature quadrupole interaction. If we assume that acid at room temperature is of the order quadrupole interaction frequency for Ta 100 MHz.

dependence of the correlation of IO-10 sec13) in HzS04 will

rc as well as the time in sulphuric the fundamental be approximately

The authors wish to express their gratitude to Professor H. N iew odniczabski for his constant interest in this work. They are also indebted to Dr. I. Stroriski for preparation of the sample and to Mr. G. Zapalski and Mr. T. Walczak for assistance in taking data. Received

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