Perturbation of the angular correlation of the 1.35 MeV-0.198 MeV ψ-γ cascade in 19F

Nuclear Physics 80 (1966) 4 7 6 - - 4 8 0 ; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

PERTURBATION OF THE ANGULAR CORRELATION

OF THE 1.35 MeV-.0.198 M e V 3'-3' C A S C A D E I N tgF H. ROPKE Physikalisches lnstitut der Unioersitiit, Freiburg/Brsg., Germany t

Received 29 October 1965 Abstract: Th~ electric and magnetic fields acting on I°F nuclei, which were accelerated into several metallic media (Cu, Sn, Pb, W, Fe, Ni) by the recoil of the nuclear reaction X*F(p,p')l°F*, were studied. Conclusions about the fields were obtained from a measurement of the perturbation of the angular correlation of the 1.35 MeV-0.198 MeV cascade of 19F. For Cu, Sn, W, Pb the perturbation is explained by quadrupole interaction of the 19F nuclei, the quadrupole frequencies vo = 0~0/2~being about 1 MHz. The perturbations in Ni and Fe are due to magnetic dipole interaction. The magnetic field at the site of the XgFnuclei is 120 kG+10 ~ in the case of Ni. E]

I

NUCLEAR REACTIONS 19F(p, p'y), E ~ 4.3 MeV; measured ~, y(O, t) Deduced angular correlation attenuation factors.

1. Introduction The angular correlation o f the 1.35 M e V - 0 . 1 9 8 MeV 7-Y cascade ~-3) in 19F which is caused by the decay o f the 1.55 MeV (J~ = ~+) level to the 0.198 Me~ (J~ --- ~+) level, followed by the ground-state transition, is suited to study the inter. action o f the 19F nuclei with the electromagnetic fields o f their environment. T N 87 ns half-life 4) o f the J~ = ~r+ state is large enough, that the population o f the m a g netic substates o f this level is considerably changed by the interaction o f the nucle with weak extranuclear fields. Following the calculations of several authors 5, 6), it i possible to draw conclusions about the interaction mechanism by observing the an gular correlations as a function o f the time between the emission o f the y-rays o f t h cascade.

2. Experimental Procedure C a F 2 targets (60/~g/cm 2) were b o m b a r d e d with 4.3 M e V protons o f the Van d Graaff accelerator o f the institute to excite the 1.55 MeV (J~ = ~t+) level in 19F b the reaction 19F(p, p')19F*. D u e to the recoil o f the reaction about 80 ~ o f the e~ cited nuclei were accelerated into various metallic targetbackings 7, ~ ) . The time dependence o f the angular correlation of the cascade was measured by the delaye t Work supported by the German Bundesministerium for wissenschaftliche Forschung av Deutsche Forschungsgemeinschaft. 476

y-y

CASCADE

IN

477

loF

coincidence method. The y-rays were detected by two NaI(TI) scintillation crystals (5.1 x 5.1 cm and 3.8 x 2.5 cm) on 56 UVP multipliers. The 1.35 MeV y-rays were measured under an angle of 270” with respect to the beam, the 0.198 MeV y-rays under angles of 42” and 90”. The pulses of the counters were fed to a time-amplitude converter (Laben model Nanocron). The conversion range of the converter was 25 ns. The output pulses of the converter were given to a multi-channel analyser and stored in different subgroups for the two locations of the 0.198 MeV y-ray detector. 3. Results In the following figures 9 is the angle of the 0.198 MeV y-ray detector with respect to the beam and Z(9) the corresponding coincidence counting rate. Fig. 1 gives the ratios of coincidence counting rates 2(42”)/2(90”) as a function o time for target backings of W, Sn, Cu, Pb. For zero delay-time between the y-rays the ratio is 2(42”)/2(90”) = 1.44& 0.04. Using this value and the multipolarities Ml for the Z(L27

iPW

1.5 1.4 1.3

1.2 1.1

Fig.

1. Ratio

of coincidence

counting

rates 2(42”)/2(90”) target backings.

as function

of time for several

metallic

478

H. R6PI~

1.35 MeV ?-ray and E2 for the 0.198 MeV ?-ray, which are given by Philips s), the unperturbed angular correlation is calculated with the aid o f the tables of Smith 9) to be W(O) = 1 +0.995 cos ~ ~ - 0 . 3 6 1 cos 4 0. (1) A measurement of the p r o m p t (A t < 15 ns) coincidences of the cascade for different values of 0 confirmed the calculation (fig. 2). -

/.3 /.2

/

-

1.1 ~

I 05

1__

I cos20

Fig. 2. Comparison of experimental and theoretical values of the unperturbed angular correlation. Because the target backings are not ferromagnetic, electric quadrupole interaction is expected to be the reason for a perturbation of the angular correlation. Therefore it is attempted to interpret the measurements of fig. 1 in a quantitative way as static interaction of the quadrupole m o m e n t of the 19F nuclei in the J~ = ~+ state with electric field gradients of random orientation. The assumption of r a n d o m orientation is justified, because the target backings were pressed from powder or made by evap. oration. Fig. 3 gives the theoretical value Z ( 4 2 ° ) / Z ( 9 0 °) as a function of mo t o~0 is defined by 090 = 6

eQV~z

4 J ( 2 J - 1)' where J and Q are spin and quadrupole m o m e n t of the nucleus in the intermediat~ state and V= the z z component of the field gradient tensor. The values of Z ( 4 2 ° ) / Z ( 9 0 ° TABLE 1

Values of the quadrupole frequencies Targetbacking Pb W Sn Cu

O)o

v0 = ~

(MHz)

1.9±10 ~o 1.4:k10 % 1.2~10 1.0-4-10

~'7 CASCADE IN leF

479

were calculated by using the formulae in 6) for the case of axial symmetry of the field gradient. For COot < ½n, which is valid for our measurements, deviations from axial symmetry give effects which are too small 6) to be detected in this experiment. A fit was made to the measurements of fig. 1 using the fact that the curve in fig. 3 is nearly a straight line for coot < ½re. The results are given in table 1. z(~2 °) theor.

I

i

1.4 1.3 1.2 1.1 1 0.9

I r/2

I 7r

~,t

27r

Fig. 3. Theoretical values o f Z ( 4 2 ° ) / Z ( 9 0 °) as a function of time for static quadrupole interaction of

axial symmetry and random orientation. z(~ z(9o.,

I

I

I

I

I

Z (9 0°)

ZTHg/cm "CoF2

Z5

54 5

1.5~

on 2[J Ni foil

,

2

2.5 H Fe foil

1.4 1.3 1.2 1.1 1

,o

20 (e)

t [~sl

10

20

t

I

30

(b)

Fig. 4. Ratio o f coincidence counting rates Z ( 4 2 ° ) / Z ( 9 0 °) as function of time for ferromagnetic target backings of Ni and Fe. The dotted line in fig. 4a indicates the damping of the oscillation.

Fig. 4 shows further measurements with target backings of thin Ni and Fe foils. The perturbations of the angular correlations are due to the strong magnetic fields of these materials. In the case of Ni the oscillation of the coincidence counting rate with time is interpreted as Larmor precession of the nuclei in fields of random orientation. (The theory is given in ref. 12)). It is necessary to notice the effect of the t91 nuclei which decay in CaFz. They are the reason that Z(42°)/Z(90 °) > 1 for times of the order 11) t < lOOns. Using the magnetic moment of the J~ = {+ state given by Freeman lo), the magnetic field strength is obtained to be 120 kG_+ 10 ~ . In the case of Fe the period of the Larmor precession is evidently smaller than the 5 ns time resolution of the time-amplitude converter. The value of Z(42°)/Z(90 °)

480

~. ROPKE

for t > 25 ns is due to the nuclei which decay in CaFz. The strong damping of th~ oscillations can be interpreted in two ways: (a) frequency distribution of the Lanno~ precession and (b) relaxation of the nuclear spin. A frequency distribution could be explained by perturbation of the lattice of Ni and Fe by the fast 19F nuclei and protons. On the other hand Klepper 1i) has made a measurement with Ni target backings of the integral anisotropy of the 0.198 MeV ?-rays with respect to the beam of the accelerator. His values were below the "hard core" value 6) of static interactions. (The "hard core" value had been calculated from a measurement of Christy ~2).) Therefore a relaxation o f the nuclear spin must be taken into account. Other authors 13) have observed Larmor precessions of 111Cd nuclei which were imbedded in a Ni lattice, for times up to 300 ns without evidence of relaxation. This makes spin-lattice relaxation improbable as the reason for out results. Improved measurements are in preparation which may give an answer tc these questions. I would like to thank Dr. H. Spehl, Mr. O. Klepper and Mr. W. Ebert for theil help during the measurements and for many stimulating discussions.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13)

B. J. Toppel, D. H. Wilkinson and D. E. Alburger, Phys. Rev. 101 (1956) 1485 J. D. Prentice, N. W. Gebbie and H. S. Caplan, Phys. Lett. 3 (1963) 201 Nuclear Data Sheets Nuclear Data Sheets A. Abragam and R. V. Pound, Phys. Rev. 92 (1953) 943 R. M. Steffen and H. Frauenfelder, in Perturbed angular correlations, ed. by E. Karlsson, l Matthias and K. Siegbahn (North-Holland Publ. Co., Amsterdam, 1964) p. 3 O. Klepper, Diplomarbeit, Freiburg (1964) unpublished W. R. Philips et aL, Phil Mag. 1 (1956) 576 P. B. Smith, in Nuclear reactions, Vol. 2, ed. by P. B. Smith and P. M. Endt (North-Hollar Publ. Co., Amsterdam, 1952) p. 248 R. A. Freeman, Nuclear Physics 26 (1961) 446 O. Klepper and H. Spehl, Nuclear Physics 64 (1965) 393 R. F. Christy, Phys. Rev. 94 (1954) 1077 E. Matthias, S. S. Rosenblum and D. A. Shirley, Phys. Rev. Lett. 14 (1965) 46