- Email: [email protected]
Lifetime of the first excited state in 29P and 29Si
Volume
PHYSICS
30B, number 2
LIFETIME
OF
P. G. BIZZETI, Istituto
fi Fisica
dell
THE
FIRST
LETTERS
EXCITED
15 September
STATE
IN
2gP
AND
1969
2gSi*
A. M. BIZZETI-SONA, A. CAMBI and P. R. MAURENZIG di Firenze, Italy Universith di Firenze, I. N.F. N., Sottosezione and
Istituto
di Fisica
dell
Universitd
C . SIGNORINI di Padova, I.N.F.N.,
Received
Sezione di Padova,
Italy
1 August 1969
The mean lives of the first excited states in 2gP and 2g Si have been measured with the Doppler shift attenuation method as ‘rrn = 0.18 f 0.03 and 0.43 * 0.05 ps, respectively. The relative importance of the isoscalar and of the two isovector terms in the MI transition amplitudes has been obtained from fl and Ml strengths in “analogous” transitions.
It has been pointed out that a simple relation exists between the strength of an Ml transition with AT = 1 in self-conjugate nuclei and the “analogue” Gamow-Teller /3 transition [1,2]. From this relationship, one can obtain information on the relative importance of the “orbital” and “spin” part of the Ml transition amplitude. In particular it has been shown [3] that experimental results require significant orbital contributions, in contrast with previous assumptions [l]. It may be observed that a similar relation can be obtained for Ml transitions in mirror nuclei, ‘where also the isoscalar part contributes. Therefore, in this case, one needs the Ml strengths of both y transitions and that of the “analogous” p decay. An interesting case can be found in the 2gSi -29P pair, where the Ml transition from the first excited state to the ground state should l-forbidden in a zero order shell model approach. The knowledge of the relative importance of the three terms in this transition amplitude can give more insight in and a more sensitive test for the rowing theoretical description of nuclei around B%i [4]. We report, in this letter, the experimental determination of the mean lives of the first excited states in 29P and 2gSi. No experimental value existed for the 2gP case; for 2gSi we preferred to repeat the mesurement since the *Work performed at the “Laboratori naro” I . N. F . N . (Padova) 94
Nqzionali
di Leg-
previous determinations [5-71 were in strong disagreement. The measurements, with the Doppler shift attenuation method, have been carried out at the 5.5 MeV accelerator of the “Laboratori Nazionali di Legnaro” (Padova). The reaction 2%i(p,y)29P at the 1.65 MeV resonance was used in the first case, with a thick Si target. In the second case the 2gSi(p, p’y)29Si reaction was used at the 1.748 MeV resonance, with a thin 29Si oxide target, evaporated on 2&i oxide. A planar 6 cm3 Ge(Li) counter z and a coaxial one of 30 cm3 volume have been used, with an energy resolution of about 3.5 keV for 1.33 MeV y rays. Both for 29P and 2gSi, y-ray spectra were recorded at four angles with respect to the beam direction, for a total of 22 runs. Typical spectra are shown in fig. 1. For the 2gP first excited state, the values E = 1383.6 f 0.1 keV and A E = 1.06 * 0.10 keV have been obtained for the unshifted energy and for the average Doppler shift. Since the calculated full Doppler shift is AEO = 2.85 keV the attenuation factor is F = A E/A E = 0.38 f 0.03. For 29Si the values E = 1273.3’* 0.1 keV, AE = = 0.65 f 0.04 keV and F = 0.25 f 0.02 have been found. The functional dependence of F on the mean life has been calculated with the usual approximations [8,9] for the slowing-down process. $ This counter was prepared by the detector group of the Istituto di Fisica dell’Universita di Firenze (contract CNR 115/238/120).
PHYSICS
Volume 30B, number 2
a few percent. In fact, in the case of 2gSi the E2/Ml mixing ratio is known [13] to correspond to 6 = 0.21 f 0.03. Since no information exists in the 2gP case, we have carried out some angular correlation measurements in the 28Si (p, y y) reaction obtaining an upper limit of 0.3 for the corresponding 161. Therefore the strengths of the two Ml transitions are M2(M1) = 0.07 W.U. for 2gP and M2(Ml) = 0.036 W.U. for 29Si. From these values, with a straightforward generalization of the Kurath formula quoted in l] and the known experimental values for the 2!! P B decay [14,15] to the first excited state of 29Si, one obtains
k!
? 1500. 2 120° 4 i =: 1000 500
'.'.'
0
II
1.59keV+
O0
1
3coo
15 September 1969
LETTERS
x 2.5
2(gE, - &)
(JOT0 IIwlq)
= g”p- 8” (JOT0Il~~llJ1~1)
.,..
5000 1000 4 "'
01.+
.
960
1090
1130 CHANNEL
The “electronic” part of the stopping power was calculated from eq. (2. 10) of ref. 8 with 51 = = 1.2 Z{for 2g P and 51 = ZIZ for 2gSi, according to the results of [lo]. The resulting mean lives are 7m = 0.18 f 0.03 ps and 0.43 f 0.05 ps for the 29P and 2g Si first excited states, respectively. The above errors do not take into account the uncertainty in the slowing-down theory for heavy ions. With a rough guess of this additional uncertainty [ll] the errors could be raised to about 20%. In the ratio of the two lifetimes, however, this error may partially cancel out. The measured lifetime in 2gSi is in fair agreement with the values obtained recently in other experiments $ while it is at variance with an earlier result obtained with the resonance fluorescence method [5]. Since there are some doubts about the earlier measurement [12], the weighted mean Tm = 0.41 f 0.04 ps of the present result and that of [6] and [7] will be adopted in what folZEs
;;‘29’;l”,“n$t%$i
but the E2 contribution
g; +gsn -g:, - g:: (JoToIl 41JlTl)
NUMBER
Fig. 1. Relevant parts of twoy-ray s ectra for Doppler shift measurements in %P.
tions,
= 0.47f 0.20(or -2.47f 0.20),
frrmth;e~;;;;;~;nsi_ is of the order of
*I.e. 7 = 0.31 f 0.10 ps from [71 and r = 0.44 * 0.10 ps fro% the I’7 value obtained by Retz-%hmidt and Morgenstern as quoted in 161.
g:
-g”,
(JJoll~~llJlTI)
= 0.12
f 0.04,
where the symbols are obvious generalizations of those of [l]. Of the two possible values for the first ratio, the first one is the most probable, due to the small coefficient in the orbital term. The quoted errors correspond to a 20% uncertainty in the stopping power of heavy ions. The present result suggests a substantial contribution of the orbital term to the Ml transition amplitude, as well as a small but significant contribution of the isoscalar term. In addition to the present case, two other pairs of mirror transitions exist (i.e. ‘Li, 7Be and 2lNe, 21N) for which the relevant experimental information [15-181 is available for analysis. The ratios of transition amplitudes are close to the above values for 21Ne, 21Na (0.39 f 0.11 and 0.12 f 0.05 respectively) while they are close to zero for PLi and “Be. It is interesting to note that for 29Si, 29P, as well as for 21Ne, 21 Na, the experimental ratio of the orbital to the spin isovector part is in remarkable agreement with the value + 0.425 expected for Ml transitions between (d+)% configuration components, with free-particle values for gyromagnetic ratios. The value expected for dl - d2 and d> -+ d; transitions would be -0.106 and -0.837 re&pectively. Obviously more configuration mixing in the two states may also account for the experimental results. It would be interesting to have detailed theoretical predictions for the relative importance of the three terms from the available complex theoretical descriptions [4]. 95
Volume 30B, number 2
PHYSICS
LETTERS
The authors are indebted to Prof. M. Mandb and R. A. Ricci for useful discussions and to Dr. R. W. Kavanagh for a kind exchange of information. A critical reading of the first version of the manuscript by Prof. P. M. Endt is also gratefully acknowledged.
7. S. I. Baker
8. 9. 10. 11. 12.
References
D. H. Yongblood, G. C. Morrison 1. A. E.Blaugrund, and R. E. Segel, Phys. Rev. 158 (1967) 893; E. F. Kennedy, D. H. Youngblood and A. E. Blaugrund, Phys. Rev. 158 (1967) 897. 2. R. W. Kavanagh, Nucl. Phys. Al29 (1969) 172. 3. D. H. Youngblood et al., Phys. Rev. 164 (1967) 1370. E. C. Halbert and P. 4. B. H. Wildenthal, J. B. McGrory, W. M. Glaudemans, Phys. Letters 27B (1968) 611; P. W. M. Glaudemans, A. E. L. Dieperink, R. J. Keddy and P.M. Endt, Phys. Letters 28B (1969) 645. 5. E. C. Booth and K. A. Wright, Nucl. Phys. 35 (1962) 472. Nu6. S. J. Skorka, J. Hertel and T. W. Retz-Schmidt, clear Data A2 (1966) 347.
*****
96
13. 14. 15. 16. 17. 18.
15 September
1969
and R. E.Segel, Phys. Rev. 170 (1969) 1046. J. Lindhard, M. Scharff and H. E.‘Schi&t, Math. Fys. Medd. Dan. Vid. Selsk. 33 No. 14 (1963). A.E.Balugrund, Nucl. Phys. 88 (1966) 501. J. H.Ormrod, J. R. MacDonald and H. E. Duckworth, Can. J. Phys. 43 (1965) 275. W. M. Currie, L.G. Earwaker and J. Martin, to be published. E. C. Booth, B. Chasan and K. A. Wright, Nucl. Phys. 57 (1964) 403. G. J. McCallum, Phys. Rev. 123 (1961) 568. 0. Lansjs, Physica Norvegica 1 (1962) 41. P. M. Endt and C. Van der Leun, Nucl. Phys. A105 (1967) 1. P. Paul, J. B. Thomas and S. S. Hanna, Phys. Rev. 147 (1966) 774. A. Bamberger, K. P. Lieb, B. Povh and D. Schwalm, Nucl. Phys. .4111 (1968) 12. F. Ajzenberg-Selove and T. Lauritsen, Nucl. Phys. 11 (1959) 1.
PHYSICS
30B, number 2
LIFETIME
OF
P. G. BIZZETI, Istituto
fi Fisica
dell
THE
FIRST
LETTERS
EXCITED
15 September
STATE
IN
2gP
AND
1969
2gSi*
A. M. BIZZETI-SONA, A. CAMBI and P. R. MAURENZIG di Firenze, Italy Universith di Firenze, I. N.F. N., Sottosezione and
Istituto
di Fisica
dell
Universitd
C . SIGNORINI di Padova, I.N.F.N.,
Received
Sezione di Padova,
Italy
1 August 1969
The mean lives of the first excited states in 2gP and 2g Si have been measured with the Doppler shift attenuation method as ‘rrn = 0.18 f 0.03 and 0.43 * 0.05 ps, respectively. The relative importance of the isoscalar and of the two isovector terms in the MI transition amplitudes has been obtained from fl and Ml strengths in “analogous” transitions.
It has been pointed out that a simple relation exists between the strength of an Ml transition with AT = 1 in self-conjugate nuclei and the “analogue” Gamow-Teller /3 transition [1,2]. From this relationship, one can obtain information on the relative importance of the “orbital” and “spin” part of the Ml transition amplitude. In particular it has been shown [3] that experimental results require significant orbital contributions, in contrast with previous assumptions [l]. It may be observed that a similar relation can be obtained for Ml transitions in mirror nuclei, ‘where also the isoscalar part contributes. Therefore, in this case, one needs the Ml strengths of both y transitions and that of the “analogous” p decay. An interesting case can be found in the 2gSi -29P pair, where the Ml transition from the first excited state to the ground state should l-forbidden in a zero order shell model approach. The knowledge of the relative importance of the three terms in this transition amplitude can give more insight in and a more sensitive test for the rowing theoretical description of nuclei around B%i [4]. We report, in this letter, the experimental determination of the mean lives of the first excited states in 29P and 2gSi. No experimental value existed for the 2gP case; for 2gSi we preferred to repeat the mesurement since the *Work performed at the “Laboratori naro” I . N. F . N . (Padova) 94
Nqzionali
di Leg-
previous determinations [5-71 were in strong disagreement. The measurements, with the Doppler shift attenuation method, have been carried out at the 5.5 MeV accelerator of the “Laboratori Nazionali di Legnaro” (Padova). The reaction 2%i(p,y)29P at the 1.65 MeV resonance was used in the first case, with a thick Si target. In the second case the 2gSi(p, p’y)29Si reaction was used at the 1.748 MeV resonance, with a thin 29Si oxide target, evaporated on 2&i oxide. A planar 6 cm3 Ge(Li) counter z and a coaxial one of 30 cm3 volume have been used, with an energy resolution of about 3.5 keV for 1.33 MeV y rays. Both for 29P and 2gSi, y-ray spectra were recorded at four angles with respect to the beam direction, for a total of 22 runs. Typical spectra are shown in fig. 1. For the 2gP first excited state, the values E = 1383.6 f 0.1 keV and A E = 1.06 * 0.10 keV have been obtained for the unshifted energy and for the average Doppler shift. Since the calculated full Doppler shift is AEO = 2.85 keV the attenuation factor is F = A E/A E = 0.38 f 0.03. For 29Si the values E = 1273.3’* 0.1 keV, AE = = 0.65 f 0.04 keV and F = 0.25 f 0.02 have been found. The functional dependence of F on the mean life has been calculated with the usual approximations [8,9] for the slowing-down process. $ This counter was prepared by the detector group of the Istituto di Fisica dell’Universita di Firenze (contract CNR 115/238/120).
PHYSICS
Volume 30B, number 2
a few percent. In fact, in the case of 2gSi the E2/Ml mixing ratio is known [13] to correspond to 6 = 0.21 f 0.03. Since no information exists in the 2gP case, we have carried out some angular correlation measurements in the 28Si (p, y y) reaction obtaining an upper limit of 0.3 for the corresponding 161. Therefore the strengths of the two Ml transitions are M2(M1) = 0.07 W.U. for 2gP and M2(Ml) = 0.036 W.U. for 29Si. From these values, with a straightforward generalization of the Kurath formula quoted in l] and the known experimental values for the 2!! P B decay [14,15] to the first excited state of 29Si, one obtains
k!
? 1500. 2 120° 4 i =: 1000 500
'.'.'
0
II
1.59keV+
O0
1
3coo
15 September 1969
LETTERS
x 2.5
2(gE, - &)
(JOT0 IIwlq)
= g”p- 8” (JOT0Il~~llJ1~1)
.,..
5000 1000 4 "'
01.+
.
960
1090
1130 CHANNEL
The “electronic” part of the stopping power was calculated from eq. (2. 10) of ref. 8 with 51 = = 1.2 Z{for 2g P and 51 = ZIZ for 2gSi, according to the results of [lo]. The resulting mean lives are 7m = 0.18 f 0.03 ps and 0.43 f 0.05 ps for the 29P and 2g Si first excited states, respectively. The above errors do not take into account the uncertainty in the slowing-down theory for heavy ions. With a rough guess of this additional uncertainty [ll] the errors could be raised to about 20%. In the ratio of the two lifetimes, however, this error may partially cancel out. The measured lifetime in 2gSi is in fair agreement with the values obtained recently in other experiments $ while it is at variance with an earlier result obtained with the resonance fluorescence method [5]. Since there are some doubts about the earlier measurement [12], the weighted mean Tm = 0.41 f 0.04 ps of the present result and that of [6] and [7] will be adopted in what folZEs
;;‘29’;l”,“n$t%$i
but the E2 contribution
g; +gsn -g:, - g:: (JoToIl 41JlTl)
NUMBER
Fig. 1. Relevant parts of twoy-ray s ectra for Doppler shift measurements in %P.
tions,
= 0.47f 0.20(or -2.47f 0.20),
frrmth;e~;;;;;~;nsi_ is of the order of
*I.e. 7 = 0.31 f 0.10 ps from [71 and r = 0.44 * 0.10 ps fro% the I’7 value obtained by Retz-%hmidt and Morgenstern as quoted in 161.
g:
-g”,
(JJoll~~llJlTI)
= 0.12
f 0.04,
where the symbols are obvious generalizations of those of [l]. Of the two possible values for the first ratio, the first one is the most probable, due to the small coefficient in the orbital term. The quoted errors correspond to a 20% uncertainty in the stopping power of heavy ions. The present result suggests a substantial contribution of the orbital term to the Ml transition amplitude, as well as a small but significant contribution of the isoscalar term. In addition to the present case, two other pairs of mirror transitions exist (i.e. ‘Li, 7Be and 2lNe, 21N) for which the relevant experimental information [15-181 is available for analysis. The ratios of transition amplitudes are close to the above values for 21Ne, 21Na (0.39 f 0.11 and 0.12 f 0.05 respectively) while they are close to zero for PLi and “Be. It is interesting to note that for 29Si, 29P, as well as for 21Ne, 21 Na, the experimental ratio of the orbital to the spin isovector part is in remarkable agreement with the value + 0.425 expected for Ml transitions between (d+)% configuration components, with free-particle values for gyromagnetic ratios. The value expected for dl - d2 and d> -+ d; transitions would be -0.106 and -0.837 re&pectively. Obviously more configuration mixing in the two states may also account for the experimental results. It would be interesting to have detailed theoretical predictions for the relative importance of the three terms from the available complex theoretical descriptions [4]. 95
Volume 30B, number 2
PHYSICS
LETTERS
The authors are indebted to Prof. M. Mandb and R. A. Ricci for useful discussions and to Dr. R. W. Kavanagh for a kind exchange of information. A critical reading of the first version of the manuscript by Prof. P. M. Endt is also gratefully acknowledged.
7. S. I. Baker
8. 9. 10. 11. 12.
References
D. H. Yongblood, G. C. Morrison 1. A. E.Blaugrund, and R. E. Segel, Phys. Rev. 158 (1967) 893; E. F. Kennedy, D. H. Youngblood and A. E. Blaugrund, Phys. Rev. 158 (1967) 897. 2. R. W. Kavanagh, Nucl. Phys. Al29 (1969) 172. 3. D. H. Youngblood et al., Phys. Rev. 164 (1967) 1370. E. C. Halbert and P. 4. B. H. Wildenthal, J. B. McGrory, W. M. Glaudemans, Phys. Letters 27B (1968) 611; P. W. M. Glaudemans, A. E. L. Dieperink, R. J. Keddy and P.M. Endt, Phys. Letters 28B (1969) 645. 5. E. C. Booth and K. A. Wright, Nucl. Phys. 35 (1962) 472. Nu6. S. J. Skorka, J. Hertel and T. W. Retz-Schmidt, clear Data A2 (1966) 347.
*****
96
13. 14. 15. 16. 17. 18.
15 September
1969
and R. E.Segel, Phys. Rev. 170 (1969) 1046. J. Lindhard, M. Scharff and H. E.‘Schi&t, Math. Fys. Medd. Dan. Vid. Selsk. 33 No. 14 (1963). A.E.Balugrund, Nucl. Phys. 88 (1966) 501. J. H.Ormrod, J. R. MacDonald and H. E. Duckworth, Can. J. Phys. 43 (1965) 275. W. M. Currie, L.G. Earwaker and J. Martin, to be published. E. C. Booth, B. Chasan and K. A. Wright, Nucl. Phys. 57 (1964) 403. G. J. McCallum, Phys. Rev. 123 (1961) 568. 0. Lansjs, Physica Norvegica 1 (1962) 41. P. M. Endt and C. Van der Leun, Nucl. Phys. A105 (1967) 1. P. Paul, J. B. Thomas and S. S. Hanna, Phys. Rev. 147 (1966) 774. A. Bamberger, K. P. Lieb, B. Povh and D. Schwalm, Nucl. Phys. .4111 (1968) 12. F. Ajzenberg-Selove and T. Lauritsen, Nucl. Phys. 11 (1959) 1.