Dynamical effects on 181Ta Nuclei in ice

Volume 4, number 5

CHEMICAL PHYSKS LETTERS

DYNAMICAL

EFFECTS

ON

181Ta

NUCLEI

J. A. CAMERON, P. I?. GARDNER, L. EESZTHELYI* McMaster

Univwsity.

15 November 1969

_Yamiitoa, Ontario,

IN ICE

and W. V. PRESTWICH Canadn

Received 23 September 1969 A temperature dependent increase of the asymmetry in the angular correlation of 18%a nudei in ice was found between - 1lOoC and - 60%. showing the lively motion of water molecules around Ta ions. In the Mijssbauer effect study of diluted frozen aqueous solution (FAS) of different ferrous and ferric salts interesting phenomena are faund[1,2]: As the temperature is increased from -1lOoC to -900C the lines broaden; near -9OOC the effect disappears and after some time reappears with different characteristics (this happens mainly in cases of ferrous salts). This shows that the Fe ions are in lively motion between -1lOoC and -70°C and in some cases their motion stops after a time. These phenomena were interpreted as the consequences of a phase transition in ice from cubic to hexagonal structure. It was assumed that the water molecules move during the phase transition agitating the impurity ions. The movement of water molecuIea was confirmed by a nuclear magnetic resonance (NMR) experiment on protons [3]. The perturbed angular correlation method (PAC) shows two advantages in such studies: 1) the number of nuclei can be extended, 2) results can be obtained in the cases when no MGssbauer-effect can be observed because of its zero probability. As a first experiment the PAC of the 133-482 keV cascade of lalTa was studied. The mean life of the 482 keV level is 7 = 15.1 nsec. The irratiated HF metal (= 10 mg) was dissolved in 27N HF acid and about 5 times more distilled water was added. The anisotropy A = (1180 - 1go)/1go was measured with the source as a liquid and, after freezing to - 140°C, as a function of temperature (fig. 1). Below - llOoC, A is equal to the hard core value [4] (M 8%) and at high temperature it gradually increases to the liquid value. The results are * On leave of absence from the Central Rescach Institute for Physics, Budapest. Hungary.

- rol

I

I

t

,

I

,

-140 -120 -lo0

I

,

-80

I

,

-60

r

I

-40

I

Fig. l_ Inte al asymmetry&&es for 133-462 keV cascade of P9lTa nuclei in FAS depending on temperature. A(Q)= G2 P). reversible. Differential measurements of A(t) were performed at - 140°C and - 90°C @ is the time after the formation of the 482 keV tevel). The results are shown in fig. 2. The PAC results are described by the equa-

where the G&(t) contain the effects of the perturbation [4]. In the cases of static quadrupole interaction G&(t) is a periodic function of w&-f, where w is the quadrupole frequency_ In time dependen$ cases when the intermediate state is affected by a randomly fluctuating environment Gk (t) = exP (+), where 3 Xk = srcw$

k(ktl)

[e(J+l)-k(k+l)-I]

(2) (3) 229

Volume 4, nnmlnx 5

CHEMICAL

PHYSICS LFTTEHS

15 November 1969

Gk

(a! = l/(1 + xkr)

(4)

in the time dependent case. In our case A is proportional to G2 to a good approximation. Fromthedata(fi .2)wg=6X107Hzand 0, (- 90°) = 2.4x10- % set can be calculated. Using (4) to interpret the data of fig. 1, rc versus temperature values are obtainable showing a variation of a factor of 30 from - llO” to - 70°C. The PAC experiment demonstrates the lively motion around the Ta ions in FAS in the temperature range found by MBssbauer effect. 7c is -~. about 500 time-bigger than ro, the jumping frequency of Fe ions calculated from line brosdening [Z]. It seems that the rapid movement of water molecules around cations in ice is a very general phenomenon. It has to be noted at the end that this experiment showed the usefulness of PAC in such dynamical studies. The authors of this manuscript acknowledge grants received from the National Research Council and the Alfred P. Sloan Foundation. REFERENCES [l] I. Wzsi, 0

4

12

8

TIME,

16

20

24

nsec

Fig. 2. Differential asymmetry values for 133-482 keV cascade of 191Ta nuclei in FAS, a) at -143OC. b) at -90°c, A(t) = c2(t).

and where rc is the average time between two appreciable interactions between the fluctuating environment and the nucleus in a state of spin J. The time integrated value is the hard core value in the case of static interaction if w T > 5,and is

Q

230

L. Keszthelyi, B. MolnlLr and L. P&s, Phys. Letters 18 (1965) 28; I. Wzsi, L. Keszthelyi, B. Molnttr and L. Pdcs ,in:

Hype&me structure and nuclear radiations, eds.

E. Matthias and D. Shirley (North-Holland, dam, 1968) p. 566. [2] $_16 Nozik and M. Kaplan, J. Chem. Phys.

Amster-

47 (1967)

[3] A. J&ossy, G. GrUner and K. Tompa, KFKI preprint, 3@ (1968). [4] H. FrauenfeIder and R.M.Steffen. in: Alpha-,beta-

and gamma-ray spectroscopy, ed. K. Siegbahn (KortkHolland, Amsterdam, 1966) p. 997.