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Magnetic hyperfine field at 206Bi in Ni
Volume 41 A, number 4
PHYSICS LETTERS
9 October 1972
M A G N E T I C H Y P E R F I N E F I E L D A T 2°6Bi IN Ni M. KAPLAN*, P.D. JOHNSTON, P. KITTEL and N.J. STONE Mullard Cryomagnetic Laboratory, Clarendon Laboratory, University of Oxford, Oxford, England Received 3 August 1972 Radioactive 2°6Bi has been dissolved in Ni and polarized at very low temperatures via magnetic hyperfine interaction. The derived magnetic hyperfine field at Bi in Ni is 390 -+ 15 kG. Similar experiments in an Fe host confirm the intractability of the system Bi/Fe. There has been considerable interest in the magnetic hyperfine fields at Bi and Pb impurities in ferromagnetic metal hosts [ 1 - 6 ] . Apparent discrepancies in results from different laboratories proved to be real and are traceable, at least in part, to differences in sample preparation [4]. In particular, sources which were prepared by implantation techniques (by menas of an isotope separator or by recoil from a preceding nuclear transmutation) yielded hyperfine fields which were inconsistent with measurements on diffused specimens. Most of these studies used gamma-gamma angular correlation techniques on Pb/Fe or Bi/Fe systems. Channeling experiments on Fe single crystals seem to indicate that the impurity location in the host lattice depends on the method of preparation and subsequent heat treatment [5]. Recent nuclear orientation measurements on 2°4Bi in Fe and Ni have led to hyperfine field values which the authors associate with substitutional sites in the host lattices [6]. From a thermodynamic point of view, Fe is a poor host for studies of Pb and Bi impurities, as these metals are very insoluble in Fe even at rather high temperatures [7]. Consequently, one might expect difficulties in sample preparation and a high sensitivity to details of heat treatment. Ni, on the other hand, displays considerable solubility for Bi [7] and should provide a more stable environment for hyperfine interaction studies. We have carried out nuclear orientation experiments on 206Bi in Ni and have measured gamma-ray anisotropies in the daughter 206pb. We report the derived magnetic hyperf'me field at 206Bi in Ni, and also on observations we have made using an Fe host. Our ex* Permanent address: Department of Chemistry, CarnegieMellon University, Pittsburgh, Pennsylvania 15213, U.S.A.
periments are similar to those of ref. [6], with the following differences: (1) We used 6.2 d 206Bi with a well-known decay scheme [8], rather than 11.3 h 204Bi; and (2) our method of preparing samples was somewhat different. Carrier-free 206Bi activity was electroplated onto Ni foils, which were sealed into quartz ampoules containing hydrogen, and melted at 1500°C. Some samples were quenched rapidly to room temperature and others were allowed to cool slowly. One sample contained three times as much Ni as the others. After cooling, each sample was carefully etched to remove any surface activity and flattened to minimize the demagnetizing field. Each in its turn, the several 2°6Bi/Ni sources were soldered to the Cu cold finger of an adiabatic demagnetization cryostat, and cooled by thermal contact with a demagnetized chrome alum reservoir. The foils were magnetized in a field of 5 kG, and sample temperatures were determined by 60Co thermometry using 60Co/Fe samples soldered adjacent to the 2°6Bi/Ni foils. The gamma rays were counted with Ge(Li) detectors and multichannel analysis at a system resolution of 2.2 keV, which provided good separation of the relevant transitions. The spectra accumulated as functions of temperature were corrected for background and radioactive decay, and the anisotropies were computed for 1 1 transitions in 206pb following the decay of 206Bi. The data obtained for the various samples above were in good agreement, and demonstrated the reproducability of the 206Bi/Ni samples. The experimental results for two transitions are shown in fig. 1. The solid curves are theoretical calculations based upon the known decay scheme [8], the known magnetic moment of 206Bi [9], and a magnetic hyperfine field Hhf = 390 kG. 315
Volume41 A, number 4
PHYSICS LETTERS
sot
.
i
206
i
°~°°
10!
o
,
i
o. o°oo
' o
I
~
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L
~
,
, _20 o -~t,' "~,, °
1720 keY ~¢"ray
30
40 ~ T'
50
~°oo _
_
60
70
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497 keV ~(ray
x ,. o
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o
r 30
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Fig. 1. Temperature dependence of 7-ray intensities in the axial direction of nuclear orientation, for the 1720 and 497 keV 7-rays emitted from 2°6Bi polarized in Ni. The 1720 keV gamma ray is pure E1 multipolarity [8] and is the only transition for which no free parameters other than the hyperfine field are involved in the fit. The temperature variation of anisotropy for all other gamma rays may be fitted with this field when due allowance for multipole admixtures is made. These will be discussed in detail elsewhere, the data on the 497 keV 7-ray [6 (E2/M1) = --0.09 (2)] being given in fig. 1 as an example. Assuming Hhf is positive and allowing for the applied field we obtain Hhf = 390 + 15 kG for the internal field at 206Bi in Ni. This value is significantly larger than the result 325 + 35 kG reported by Bacon et al. [6] from measurements on 204Bi/Ni and the discrepancy is outside the range of assigned errors. It may be that the lower value is related to the method of sample preparation used, or it may simply reflect larger uncertainties than assigned by them. For comparative purpose, we carried out studies with 206Bi/Fe sources, preparing the samples in the same way as for Ni except heating to 1550 ° C in order to melt the Fe. We found unexpectedly small gammaray anisotropies when the sources were cooled slowly, and obtained large anisotropies only when the sources were quenched very rapidly from the molten state. 316
9 October 1972
Even then, the data could only be fitted by allowing a variable "population factor" for the occupancy of " g o o d " sites and very good fits were not obtained if a unique hyperfine field was assumed. The approximate fits yielded results in the range Hhf = 8 0 0 - 1000 kG for 206Bi in Fe, compared with the reported value 1180_+ 130 [6]. In conclusion, we believe that reliable studies can be carried out in the Bi/Ni system, but that results obtained for Bi/Fe are still in some doubt. It should be noted that the argument advanced by Bacon et at. [6] in favor of substitutional sites is not entirely valid. This is because the DPAC measurement shown by them reflects only those nuclei which experience a significant hyperfine interaction, whereas the nuclear orientation data represent an average over all sites, including any in zero effective field. Similarly, our result for the magnetic hyperfine field in 206Bi/Ni will be lowered in the unlikely event of any admixture of M2 multipolarity in the 1720 keY E1 gamma ray. We have attempted to observe nuclear magnetic resonance in 206Bi/Ni, but without success on either rolled polycrystalline foils or single crystals of Ni into which the Bi was diffused. The reason may have been either a combination of fast relaxation times and a large degree of inhomogeneous broadening, or an electric quadrupole interaction smearing out the resonance from the many substate transitions in 206Bi. This work was supported by a grant from the Science Research Council which is gratefully acknowledged. One of us (MK) wishes to express his appreciation for the hospitality shown him by the Clarendon Laboratory during his stay.
References [1] G.C. Pramila, S.G. Cohen and L. Grodzins, Phys. Lett. 24A (1967) 7. [2] J.D. Bowman and F.C. Zawislak, Nucl. Phys. A138 (1969) 90. [31 F.C. Zawislak and D.D. Cook, Bull. Am. Phys. Soc. 14 (1969) 1171; preprint and private communicatiom 1972. [4] E.N. Kaufmann, Phys. Lett. 35A (1971) 165. [5] L.C. Feldman et al., Phys. Rev. Letters 27 (1971) 1145. [6] F. Bacon et al.,Phys. Lett. 38A (1972) 401. [7 ] M. Hansen, Constitution of Binary Alloys (McGraw-Hill Book Co., Inc., New York, 1958). [8] M.J. Martin, Nuclear Data Sheets B5 (1971) 179. [9] V.S. Shirley, in: Hyperfine interactions in excited nuclei,Vol. 4, eds. G. Goldring and R. Kalish (Gordon and Breach, Inc., New York, 1971) p. 1255.
PHYSICS LETTERS
9 October 1972
M A G N E T I C H Y P E R F I N E F I E L D A T 2°6Bi IN Ni M. KAPLAN*, P.D. JOHNSTON, P. KITTEL and N.J. STONE Mullard Cryomagnetic Laboratory, Clarendon Laboratory, University of Oxford, Oxford, England Received 3 August 1972 Radioactive 2°6Bi has been dissolved in Ni and polarized at very low temperatures via magnetic hyperfine interaction. The derived magnetic hyperfine field at Bi in Ni is 390 -+ 15 kG. Similar experiments in an Fe host confirm the intractability of the system Bi/Fe. There has been considerable interest in the magnetic hyperfine fields at Bi and Pb impurities in ferromagnetic metal hosts [ 1 - 6 ] . Apparent discrepancies in results from different laboratories proved to be real and are traceable, at least in part, to differences in sample preparation [4]. In particular, sources which were prepared by implantation techniques (by menas of an isotope separator or by recoil from a preceding nuclear transmutation) yielded hyperfine fields which were inconsistent with measurements on diffused specimens. Most of these studies used gamma-gamma angular correlation techniques on Pb/Fe or Bi/Fe systems. Channeling experiments on Fe single crystals seem to indicate that the impurity location in the host lattice depends on the method of preparation and subsequent heat treatment [5]. Recent nuclear orientation measurements on 2°4Bi in Fe and Ni have led to hyperfine field values which the authors associate with substitutional sites in the host lattices [6]. From a thermodynamic point of view, Fe is a poor host for studies of Pb and Bi impurities, as these metals are very insoluble in Fe even at rather high temperatures [7]. Consequently, one might expect difficulties in sample preparation and a high sensitivity to details of heat treatment. Ni, on the other hand, displays considerable solubility for Bi [7] and should provide a more stable environment for hyperfine interaction studies. We have carried out nuclear orientation experiments on 206Bi in Ni and have measured gamma-ray anisotropies in the daughter 206pb. We report the derived magnetic hyperf'me field at 206Bi in Ni, and also on observations we have made using an Fe host. Our ex* Permanent address: Department of Chemistry, CarnegieMellon University, Pittsburgh, Pennsylvania 15213, U.S.A.
periments are similar to those of ref. [6], with the following differences: (1) We used 6.2 d 206Bi with a well-known decay scheme [8], rather than 11.3 h 204Bi; and (2) our method of preparing samples was somewhat different. Carrier-free 206Bi activity was electroplated onto Ni foils, which were sealed into quartz ampoules containing hydrogen, and melted at 1500°C. Some samples were quenched rapidly to room temperature and others were allowed to cool slowly. One sample contained three times as much Ni as the others. After cooling, each sample was carefully etched to remove any surface activity and flattened to minimize the demagnetizing field. Each in its turn, the several 2°6Bi/Ni sources were soldered to the Cu cold finger of an adiabatic demagnetization cryostat, and cooled by thermal contact with a demagnetized chrome alum reservoir. The foils were magnetized in a field of 5 kG, and sample temperatures were determined by 60Co thermometry using 60Co/Fe samples soldered adjacent to the 2°6Bi/Ni foils. The gamma rays were counted with Ge(Li) detectors and multichannel analysis at a system resolution of 2.2 keV, which provided good separation of the relevant transitions. The spectra accumulated as functions of temperature were corrected for background and radioactive decay, and the anisotropies were computed for 1 1 transitions in 206pb following the decay of 206Bi. The data obtained for the various samples above were in good agreement, and demonstrated the reproducability of the 206Bi/Ni samples. The experimental results for two transitions are shown in fig. 1. The solid curves are theoretical calculations based upon the known decay scheme [8], the known magnetic moment of 206Bi [9], and a magnetic hyperfine field Hhf = 390 kG. 315
Volume41 A, number 4
PHYSICS LETTERS
sot
.
i
206
i
°~°°
10!
o
,
i
o. o°oo
' o
I
~
"> I > ~0t I
L
~
,
, _20 o -~t,' "~,, °
1720 keY ~¢"ray
30
40 ~ T'
50
~°oo _
_
60
70
K"
497 keV ~(ray
x ,. o
!
o
r 30
o
o
4@ o o
Fig. 1. Temperature dependence of 7-ray intensities in the axial direction of nuclear orientation, for the 1720 and 497 keV 7-rays emitted from 2°6Bi polarized in Ni. The 1720 keV gamma ray is pure E1 multipolarity [8] and is the only transition for which no free parameters other than the hyperfine field are involved in the fit. The temperature variation of anisotropy for all other gamma rays may be fitted with this field when due allowance for multipole admixtures is made. These will be discussed in detail elsewhere, the data on the 497 keV 7-ray [6 (E2/M1) = --0.09 (2)] being given in fig. 1 as an example. Assuming Hhf is positive and allowing for the applied field we obtain Hhf = 390 + 15 kG for the internal field at 206Bi in Ni. This value is significantly larger than the result 325 + 35 kG reported by Bacon et al. [6] from measurements on 204Bi/Ni and the discrepancy is outside the range of assigned errors. It may be that the lower value is related to the method of sample preparation used, or it may simply reflect larger uncertainties than assigned by them. For comparative purpose, we carried out studies with 206Bi/Fe sources, preparing the samples in the same way as for Ni except heating to 1550 ° C in order to melt the Fe. We found unexpectedly small gammaray anisotropies when the sources were cooled slowly, and obtained large anisotropies only when the sources were quenched very rapidly from the molten state. 316
9 October 1972
Even then, the data could only be fitted by allowing a variable "population factor" for the occupancy of " g o o d " sites and very good fits were not obtained if a unique hyperfine field was assumed. The approximate fits yielded results in the range Hhf = 8 0 0 - 1000 kG for 206Bi in Fe, compared with the reported value 1180_+ 130 [6]. In conclusion, we believe that reliable studies can be carried out in the Bi/Ni system, but that results obtained for Bi/Fe are still in some doubt. It should be noted that the argument advanced by Bacon et at. [6] in favor of substitutional sites is not entirely valid. This is because the DPAC measurement shown by them reflects only those nuclei which experience a significant hyperfine interaction, whereas the nuclear orientation data represent an average over all sites, including any in zero effective field. Similarly, our result for the magnetic hyperfine field in 206Bi/Ni will be lowered in the unlikely event of any admixture of M2 multipolarity in the 1720 keY E1 gamma ray. We have attempted to observe nuclear magnetic resonance in 206Bi/Ni, but without success on either rolled polycrystalline foils or single crystals of Ni into which the Bi was diffused. The reason may have been either a combination of fast relaxation times and a large degree of inhomogeneous broadening, or an electric quadrupole interaction smearing out the resonance from the many substate transitions in 206Bi. This work was supported by a grant from the Science Research Council which is gratefully acknowledged. One of us (MK) wishes to express his appreciation for the hospitality shown him by the Clarendon Laboratory during his stay.
References [1] G.C. Pramila, S.G. Cohen and L. Grodzins, Phys. Lett. 24A (1967) 7. [2] J.D. Bowman and F.C. Zawislak, Nucl. Phys. A138 (1969) 90. [31 F.C. Zawislak and D.D. Cook, Bull. Am. Phys. Soc. 14 (1969) 1171; preprint and private communicatiom 1972. [4] E.N. Kaufmann, Phys. Lett. 35A (1971) 165. [5] L.C. Feldman et al., Phys. Rev. Letters 27 (1971) 1145. [6] F. Bacon et al.,Phys. Lett. 38A (1972) 401. [7 ] M. Hansen, Constitution of Binary Alloys (McGraw-Hill Book Co., Inc., New York, 1958). [8] M.J. Martin, Nuclear Data Sheets B5 (1971) 179. [9] V.S. Shirley, in: Hyperfine interactions in excited nuclei,Vol. 4, eds. G. Goldring and R. Kalish (Gordon and Breach, Inc., New York, 1971) p. 1255.