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The 661 keV resonance of the Mg26(p,γ) reaction
PHYSICS
Volume ,2, number 4
15 September 1962
LETTERS
shift). While such measurements support the ideas presented above, experimental difficulties preclude high precision measurements. We wish to acknowledge valuable discussions with W. Visscher, D. E. Nagle, W. Overton and R. D. Taylor. W. Shlaer performed a number of machine computations, and R. D. Taylor, H. Coburn and John Engelhardt helped with many of the measurements. References
1) H. Frauenfelder , The M&&auer effekt (w . A. Benjamin, Inc., 1962).
9) P. P. Craig, R.D. Taylor and D. E. Nagle, Nuovo Cimento 22 (1961) 402. To he published. :; J.Petzold, Theorie des Mbssbauer-Effektes (Sitzber. Heidelberger Akad. Wiss., Math.-Naturwiss. Klasse, 1960). 5) B.Kaufman and H. Lipkln. Ann. Phys. (U.S.A.) 18 (1962) 294. W. M.Vissoher, The Msssbauer effect, D.M. J. ‘) Compton and A. H.Schoen, eds. (John Wiley and Sons, 1962) p. 27; and unpublished. 7) B.S. Chanch-asekharand J. A. Rayne, Phys. Rev. L8tters 6 (1961) 3. 8) W. Hume-Rothery and G. V. Raynor, The structure of metals and alloys, 3rd edition (‘The Institute of Metals, 1954) p. 62.
*****
THE
661
keV
RESONANCE
OF
THE
Mg26(p,y)
REACTION
T. R. OPHEL and B. T. LAWERGREN Research School of Physical Sciences, Australian National University, Canberra, Australia Received 20 August 1962 Several recent papers have discussed the experimental evidence concerning the spin of the 2.21 MeV level of Al27 l-4). Though this level is considered to be the second member of the K = %+ rotational band, some measurements conflict with an assignment of J = f’ for the level. In particular, Van der Leun et al. 5) reported a relatively strong transition to the 2.21 MeV level at the 661 keV resonance (J = i) of the Mg26(p,y) reaction. Since these data were obtained with a small scintillation spectrometer and contain several inconsistencies with regard to the de-excitation of the low-lying levels of A127, the gamma ray spectrum at the 661 keV resonance has been remeasured. A thin (- 3 keV) magnesium target, on a backing of 0.05 cm thick platinum, was bombarded with protons from the Canberra 1.2 MeV CockcroftWalton accelerator. The target was prepared by heating separated Mg26G * on a tantalum ribbon in vacua 6). An unshielded 12.7 cm X 10.2 cm NaI(T1) scintillation spectrometer, located at a distance of 10 cm from the target, was used as the gamma ray detector. Gamma ray line shapes and energy calibrations of the spectrometer were obtained from measurements of the gamma ray spectra from the Flg(p,oy) and C13(p,y) reactions and a ThC” source. The spectrum observed at I$ = 663 keV, cor* Suppliedby the Atomic Energy Research Establishment, Harwell, England.
rected for room background and the target background measured several kilovolts below the resonance is shown in fi . 1. Since the resonance radiatio; is isotropic 5y, measurements were made at one angle only (900). The constituent gamma rays of the spectrum, which were obtained by line shape analysis, are summarised in table 1. Gamma rays with energies of 0.84 and 1.01 MeV were also observed, but their intensities were not measured. Thus there is no evidence (upper limit 2%) for a transition via the 2.21 MeV level. The gamma rays of energies 7.16 and 6.12 MeV are attributed to a contaminant contribution from the 669 keV resonance of the Flg(p,ccy) reaction. This is confirmed by the relative intensities of the gamma rays, the measured yield variation of the gamma rays with proton energy and the absence of 2.21 and 3.0 MeV cascade radiation which would be present if the gamma rays resulted from the Mg26(p, y) reaction. It is considered that the transitions via the 2.21 and (2.98, 3.00) MeV levels reported by Van der Leun et al. can be explained by similar contamination. An energy discrepancy between 6.65 and 7.14 MeV (and 5.86, 6.14) can result from the use of a small spectrometer (4.1 cm x 5.1 cm). In such a spectrometer, the fraction of counts appearing in the full energy peak is sufficiently small that, where the spectrum contains two gamma rays differing in energy by about 1 MeV, the one escape peak can be erroneously interpreted as the full energy peak. The systematic error introduced into the line shape 167
Volume 2, number 4
PHYSICS
LETTERS
15 Septembr
1962
Table 1 Gamma rays observed at the 661 keV resonance. Present measurement Ey (MeV)
Interpretation
8.90 * 0.05 7.92 f 0.05 7.16 + 0.05 6.70 6.12 zt 0.05 5.25 f: 0.05 5.0 f 0.15 ** 3.21 LIZ 0.05 2.80 zt 0.05 2.21 1.01 f 0.03 0.84 f 0.03
r-0 Y - 1.01 F1’@, ory) r - 2.21 F1’@,ary) r - 3.68 Y * 4.05 4.05 - 0.84 3.68 - 0.84 2.21 - 0 1.01 4 0 0.84 - 0
Van der Leun et al. 5) Relative intensity cl* 100 10 c3 41 44 10 9 55 Cl
Ey (MeV)
Interpretation
7.84 f 0.10
r - 1.01
6.63 5.86 5.25 4.95
Y r r Y
+ 0.10 zt 0.08 zt 0.06 + 0.08
* -
2.21 2.98, 3.00 3.68 3.95 I:
Re dative intensity ** * 100
l
15
22 + 12 54 f 12 41 l 10 27 zt 10
* Corrected for sum pulses. ** Large error due to line shape uncertainty. *** NormaUsed so that 17.84 = 100. Fig. 1. The gamma ray spectrum at the 661 keV resonance and the composite gamma ray line shapes. The open circles represent residual counts (after the line shapes of higher energy gamma rays have been subtracted) in the regions where the gamma rays corresponding to a transition via the 2.21 MeV level would be evident.
analysis will affect the energy of the lower energy gamma ray. The absence of a transition via the 2.21 MeV level at this resonance means that all gamma ray measurements are now consistent with an assignment of J = I+* the angular distribution measurements of ine’%tically scattered protons leading to the level, which have been interpreted to indicate that the level has negative parity 81, are the remaining conflicting evidence. The cooperation of Dr. D. F. Hebbard and G. M. Bailey, who made available much of their experimental equipment for this measurement, fully aclmowledged.
is grate-
References 1) E. Almqvist et al., Nuclear Phys . 19 (1960) 1. 2) R.D. Bent and W. W.Eidson, Phys. Rev. 122 (1969
2
3
4 GAMMA
5 RAY
6 ENERGY--MeV
7
8
1514. 3) B. T. Lawergren and T.R.Ophel, Proc. Phys. Sot. 79 (1962) 881. 4) P.M. Endt and C. Van der Leun, Nuclear Phys. 34 (1962) 1. 5) C. Van der Leun et al., Physica 22 (1956) 1223. 6) L. N.Russell, W. E. Taylor and J. N. Cooper, Rev. Sci. Instr. 23 (1952) 764. 7) F. Ajzenberg-Selove and T. Lauritsen, Nuclear Phys. .I 11 (1959) 1. F.de S.Barros et al., Proc. Phys. Sot. 73 (1959) 793. 8)
Volume ,2, number 4
15 September 1962
LETTERS
shift). While such measurements support the ideas presented above, experimental difficulties preclude high precision measurements. We wish to acknowledge valuable discussions with W. Visscher, D. E. Nagle, W. Overton and R. D. Taylor. W. Shlaer performed a number of machine computations, and R. D. Taylor, H. Coburn and John Engelhardt helped with many of the measurements. References
1) H. Frauenfelder , The M&&auer effekt (w . A. Benjamin, Inc., 1962).
9) P. P. Craig, R.D. Taylor and D. E. Nagle, Nuovo Cimento 22 (1961) 402. To he published. :; J.Petzold, Theorie des Mbssbauer-Effektes (Sitzber. Heidelberger Akad. Wiss., Math.-Naturwiss. Klasse, 1960). 5) B.Kaufman and H. Lipkln. Ann. Phys. (U.S.A.) 18 (1962) 294. W. M.Vissoher, The Msssbauer effect, D.M. J. ‘) Compton and A. H.Schoen, eds. (John Wiley and Sons, 1962) p. 27; and unpublished. 7) B.S. Chanch-asekharand J. A. Rayne, Phys. Rev. L8tters 6 (1961) 3. 8) W. Hume-Rothery and G. V. Raynor, The structure of metals and alloys, 3rd edition (‘The Institute of Metals, 1954) p. 62.
*****
THE
661
keV
RESONANCE
OF
THE
Mg26(p,y)
REACTION
T. R. OPHEL and B. T. LAWERGREN Research School of Physical Sciences, Australian National University, Canberra, Australia Received 20 August 1962 Several recent papers have discussed the experimental evidence concerning the spin of the 2.21 MeV level of Al27 l-4). Though this level is considered to be the second member of the K = %+ rotational band, some measurements conflict with an assignment of J = f’ for the level. In particular, Van der Leun et al. 5) reported a relatively strong transition to the 2.21 MeV level at the 661 keV resonance (J = i) of the Mg26(p,y) reaction. Since these data were obtained with a small scintillation spectrometer and contain several inconsistencies with regard to the de-excitation of the low-lying levels of A127, the gamma ray spectrum at the 661 keV resonance has been remeasured. A thin (- 3 keV) magnesium target, on a backing of 0.05 cm thick platinum, was bombarded with protons from the Canberra 1.2 MeV CockcroftWalton accelerator. The target was prepared by heating separated Mg26G * on a tantalum ribbon in vacua 6). An unshielded 12.7 cm X 10.2 cm NaI(T1) scintillation spectrometer, located at a distance of 10 cm from the target, was used as the gamma ray detector. Gamma ray line shapes and energy calibrations of the spectrometer were obtained from measurements of the gamma ray spectra from the Flg(p,oy) and C13(p,y) reactions and a ThC” source. The spectrum observed at I$ = 663 keV, cor* Suppliedby the Atomic Energy Research Establishment, Harwell, England.
rected for room background and the target background measured several kilovolts below the resonance is shown in fi . 1. Since the resonance radiatio; is isotropic 5y, measurements were made at one angle only (900). The constituent gamma rays of the spectrum, which were obtained by line shape analysis, are summarised in table 1. Gamma rays with energies of 0.84 and 1.01 MeV were also observed, but their intensities were not measured. Thus there is no evidence (upper limit 2%) for a transition via the 2.21 MeV level. The gamma rays of energies 7.16 and 6.12 MeV are attributed to a contaminant contribution from the 669 keV resonance of the Flg(p,ccy) reaction. This is confirmed by the relative intensities of the gamma rays, the measured yield variation of the gamma rays with proton energy and the absence of 2.21 and 3.0 MeV cascade radiation which would be present if the gamma rays resulted from the Mg26(p, y) reaction. It is considered that the transitions via the 2.21 and (2.98, 3.00) MeV levels reported by Van der Leun et al. can be explained by similar contamination. An energy discrepancy between 6.65 and 7.14 MeV (and 5.86, 6.14) can result from the use of a small spectrometer (4.1 cm x 5.1 cm). In such a spectrometer, the fraction of counts appearing in the full energy peak is sufficiently small that, where the spectrum contains two gamma rays differing in energy by about 1 MeV, the one escape peak can be erroneously interpreted as the full energy peak. The systematic error introduced into the line shape 167
Volume 2, number 4
PHYSICS
LETTERS
15 Septembr
1962
Table 1 Gamma rays observed at the 661 keV resonance. Present measurement Ey (MeV)
Interpretation
8.90 * 0.05 7.92 f 0.05 7.16 + 0.05 6.70 6.12 zt 0.05 5.25 f: 0.05 5.0 f 0.15 ** 3.21 LIZ 0.05 2.80 zt 0.05 2.21 1.01 f 0.03 0.84 f 0.03
r-0 Y - 1.01 F1’@, ory) r - 2.21 F1’@,ary) r - 3.68 Y * 4.05 4.05 - 0.84 3.68 - 0.84 2.21 - 0 1.01 4 0 0.84 - 0
Van der Leun et al. 5) Relative intensity cl* 100 10 c3 41 44 10 9 55 Cl
Ey (MeV)
Interpretation
7.84 f 0.10
r - 1.01
6.63 5.86 5.25 4.95
Y r r Y
+ 0.10 zt 0.08 zt 0.06 + 0.08
* -
2.21 2.98, 3.00 3.68 3.95 I:
Re dative intensity ** * 100
l
15
22 + 12 54 f 12 41 l 10 27 zt 10
* Corrected for sum pulses. ** Large error due to line shape uncertainty. *** NormaUsed so that 17.84 = 100. Fig. 1. The gamma ray spectrum at the 661 keV resonance and the composite gamma ray line shapes. The open circles represent residual counts (after the line shapes of higher energy gamma rays have been subtracted) in the regions where the gamma rays corresponding to a transition via the 2.21 MeV level would be evident.
analysis will affect the energy of the lower energy gamma ray. The absence of a transition via the 2.21 MeV level at this resonance means that all gamma ray measurements are now consistent with an assignment of J = I+* the angular distribution measurements of ine’%tically scattered protons leading to the level, which have been interpreted to indicate that the level has negative parity 81, are the remaining conflicting evidence. The cooperation of Dr. D. F. Hebbard and G. M. Bailey, who made available much of their experimental equipment for this measurement, fully aclmowledged.
is grate-
References 1) E. Almqvist et al., Nuclear Phys . 19 (1960) 1. 2) R.D. Bent and W. W.Eidson, Phys. Rev. 122 (1969
2
3
4 GAMMA
5 RAY
6 ENERGY--MeV
7
8
1514. 3) B. T. Lawergren and T.R.Ophel, Proc. Phys. Sot. 79 (1962) 881. 4) P.M. Endt and C. Van der Leun, Nuclear Phys. 34 (1962) 1. 5) C. Van der Leun et al., Physica 22 (1956) 1223. 6) L. N.Russell, W. E. Taylor and J. N. Cooper, Rev. Sci. Instr. 23 (1952) 764. 7) F. Ajzenberg-Selove and T. Lauritsen, Nuclear Phys. .I 11 (1959) 1. F.de S.Barros et al., Proc. Phys. Sot. 73 (1959) 793. 8)