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MÖSSBAUER STUDY OF ELECTROMAGNETIC
PHENOMENA IN LOW TEMPERATURE COPPER
G. Perlow, W. Potzel, W. Koch
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, De'cembre 1976, page C6-427
MOSSBAUER STUDY OF ELECTROMAGNETIC
PHENOMENA
IN LOW TEMPERATURE COPPER
(*)
G. J. PERLOW (**), W. POTZEL and W. KOCH
Physik Department Technische Universitat Miinchen, Munich, Germany and Argonne National Laboratory, Argonne, Illinois 60439, U. S . A.
RCsumC.
-
L'emission Mossbauer B basse temperature d'une feuille de cuivre doppee au 57C0 est absorb& de faqon resonnante par du Be(57Fe) stationnaire. L'utilisation d'un courant radio- frequence de 700 MHz envoy6 B travers la source entraine une augmentation importante de la transmission, principalement B la fin de l'impulsion. Ce phenomhne est attribue B un mouvement oscillatoire dans le plan de la feuille 9. une frequence plus petite que 10 MHz. Ce phenomhne ne depend pas de la frequence du courant r-f et n'apparait pas pendant la pulsation r-f. Les relations avec d'autres processus bien connus de production d'ultrason ne sont pas imrnkdiates.Abstract. - Mossbauer radiation from a low temperature copper foil doped with 57C0 is resonantly obsorbed in a stationary Be(57Fe) absorber. Radio frequency current pulses at 700 MHz through the source cause large increases in transmission, mainly after the end of the pulse. The phenomena is attributed to ascillatory motion in the place of the foil and with motional fraquencies
< 10 MHz. As it occurs neither with the r-f current frequencies nor mainly during the r-f pulse,
its connection with other well-studied processes of direct production of ultrasound is not immediate.In experiments motivated by observations of the direct production of ultra-sound by radio frequency radiation [I], r-f current pulses at 700 MHz were passed through a 1.5 ym thick Cu foil doped with 57Co. The 14.4 keV y-radiation was transmitted through a Be disk doped with 57Fe and counted. The Mossbauer resonance of the Be(Fe) absorber is well enough described by a Lorentzian of width
r
= 0.83 mm/s/ depth a = 0.62 before background correction, and isomer shift almost exactly that of the Cu source. The foil was mounted at 45O to the direction of observation, and formed the termination at one end of a A/2 cavity. For most experiments the source and absorber were immersed in 4.2 K liq He. Some experiments were done as low as 1.68 K and, in He gas, at various temperatures up to 77 K.The ultrasonic phenomenon that we searched for would result in f-m sidebands well outside the absorp- tion line of Be(Fe) and hence would lead to the effect of an increase of count rate. In first experiments with equal times of rr r-f on D and cr r-f off H, effects up to 2
%
were seen. This was in fact larger than a calculation by Rodriguez (I) had indicated that we might expect. As
the effect increased with power, we went to pulse operation, and higher peak power. The pulsing and counting were controlled by logic circuits that per-
(*) Work supported in part by the U. S. Energy Research and Development Administration.
(**) A. v. Humboldt-Awardee 1974-1975, Technical Univer- sity, Munich.
(1) Rodriguez, S., Private communication.
mitted duty-cycles of l/n, where n was a precise integer. When plotted, the ratio R = Counts with r-f
on/(Counts with r-f off/@
-
I)), varied from 1.00 at very low power to a surprising 1.35 at our maximum power of about 4.5 W into the source. The effect depended on duty cycle and reduced to the earlier small effects at n w 2 (i. e. equal on and off times). An experiment in which the foil was turned to lie perpendicular to the direction of observation gave a small result, showing that the effect was equivalent to motion in the plane of the foil.An experiment was done with the Be(Fe) absorber removed, and a Sodium Ferrocyanide absorber moved with an external drive below the cryostat. Spectra were obtained during the pulses (whose length for this experiment was 512 ps) and for comparison with r-f shut off. We found that
r
,,,,,
=r,,
= 0.97 mm/s, a,, = 0.061, whiler,,,
= 0.30 mm/s, a,, = 0.205. No relative shift was observed. The areas are nearly iden- tical, which implies that the side-band production was unobservably small, and hence that direct production of ultra-sound is by no means responsible for the large effects observed.In the next set of experiments, with the Be(Fe) absorber back in place, we examined by multiscaling the count rate in equal intervals following the pulse up to the time of the next pulse. The time dependence of the count rate was interesting. For short pulses, less than 16 ys in length, the count rate rose from its value at the end of the pulse and reached a maximum about 100 ys later. It then decayed during the next millisecond
C6-428 G. J. PERLOW, W. POTZEL AND W. KOCH
or two to its quiescent value N(o3). Calling the effect E = (N(t)
-
N(m))/(N(m)), one finds values like E w 0.7 at the peak of the curve. The maximum possible is Em,, z 1.6. Similar data display remarkably different behaviour at lower and at higher temperatures. We have made one analysis on the basis that we are observing . a low temperature thermal expansion phenomenon and the subsequent cooling. We can convert N(t) into a speed(
v(t)I
and if we assume that ualways has the same sign during the cooling period, the hypothesis can be checked by integrating u(t) to obtain the shrinkage and comparing this with what the Grii- neisen relation predicts. Our discrepancy is two orders of magnitude. However an assumption of oscillatory motion in the plane of the Cu foil would be consistent with all of our observations to date. On the basisIof line-width, we would conclude that the oscillation frequencies are
<
10 MHz.Refereeces
[I] See e. g., QUINN, J . J., Phys. Lett. 25A (1967) 522.
CHIMENTI, D. E., KUKKONEN, C. A. and MAXFIELD, B. W., Phys. Rev. B 10 (1974) 3228.
