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The lifetime of some levels in 187Re
PHYSICS
Volume 20, number 5
15 March 1966
LETTERS
@lb) L’essentiel de la correction due a T(l) aux On a preserve dans l’ecriture de (llb) la reciprocite. grands angles de diffusion peut alors 6tre obtenu en considerant une diffusion Blastique ri petit angle sur le deuton suivi d’un “pick-up” et vice-versa. Des calculs sont actuellement en tours pour &valuer les corrections dues a T(l). R&f&wnces
1. G.Takeda et K.M.Watson, Phys.Rev.97 (1955) 1336.
2. H.Kottler et K.L.Kowalski, Phys.Rev.138 (1965) B619. 3. P. Benoist-Gueutal et F. Gomez-Gimeno, J. Phys. 26 (1965) 403.
4. 5. 6. 7.
K.L.Kowalski, Nuovo Cimento 30 (1963) 266. A.Everett, Phys.Rev.126 (1962) 831. N.M.Queen, Nuclear Phys.55 (1964) 177. P.F.Koehler, E.H.Thorndike et A.H.Cromer,
Phys.Rev.134
(1964) 1030.
*****
THE
LIFETIME
OF SOME
K. PETERSEN, G. TRUWY Laboratoy,
of Applied
Physics
LEVELS
IN 187Re
and P. @STERG~
II, Technical
University
of Denmark
Received 18 February 1966 Using the delayed coincidence method the lifetime of the 686 keV and 618 keV levels has been measured. The lifetime of the 625 keV level is discussed.
The level structure of 187Re has been investigated by many authors. For references, see Bisgaard et al. [l]. As 187Re is situated in the transition region of the deformed nuclei one may expect deviations from the strong coupling scheme. Lifetime determinations of the excited states in 187Re are thus of interest, especially of the 686 keV level, which according to Gallagher et al. [2] might be the K = G - 2 gammavibrational level built on the 206 keV level. As the lifetime measurements [3-51 on the 686 keV level do not agree we have tried to measure the lifetime with the delayed coincidence method. We have used a normal fast-slow system, which for the prompt coincidences in 60~0 gives a resolving time of 0.8 ns and a slope of 0.14 ns. One of the counters detected the beta particles in the energy region 200 - 250 keV and the other counter detected gamma radiation with an energy of about 700 keV using a window of 60 keV. The detectors were NE 102 A plastic scintillators mounted on Philips 56 AVP photomultiplier tubes. In the gamma detector the scintillator was 25 mm thick and in the beta detector the scintillator was 2 mm thick. The source was prepared by evaporation of
Table 1 Lifetime of the 686 keV level 0.29 10.10 8.5 i0.7 0.41 * 0.20 CO.2
Vartapetian [3] Langhoff [4] Shubny et al. [5] This work
ns pa ns ns
natural tungsten on aluminium foil, and the neutron irradiation was performed in DR 2 at Risd. The time spectrum, shown in fig. 1, is complex. It places an upper limit on the lifetime of the 686 keV level of 0.2 ns. In table 1 this result is compared with the earlier measurements [3-51. Table 2 Transition probabilities for the 618 keV and 625 keV levels. The experimental results Tern are compared with the Weisskopf estimates TWeiss T exp
618 keV (Ml+E2) 625 keV (E2) *
Texp /TWeiss
(1.3 hO.1) x 10’ s-l 8.2 xlO1l
s-l
1.9 x 10’4 1.1 x 102
* Using Langhoffs [4] value. See text.
527
Volume 20, number 5
PHYSICS
LETTERS
A component with a lifetime of 0.76 ns is also present in the time spectrum. (This may explain the lifetime measured by Vartapetian [3].) We have measured the ratio of the intensity of this component to the intensity of the prompt coincidences (from l5 the 686 keV level) as function of the gamma energy - and compared it with a calculated curve based upon the assumption that the lifetime must be ascribed to the 618 keV level (fig. 2). Although the uncertainties on lo both curves are great (25%), a comparison of the two curves shows that the lifetime of 0.76 ns belongs to the 618 keV transition and not to the 625 keV transition as the intensity of this is + of the intensity of the 618 keV transition. If we compare this result with Langhoff’s [4] measurements on the level width of the 618 keV + 625 keV transitions 500 (14 r. 618 keV + ro 625 keV = 5.5 x 1O-4 eV) 4oo we get a lifetime of 1.2 ps for the 625 keV level. In table 2 are aiven the measured Fig. 2. Ratio (A) transition probabilities and these are comwith the
Weisskopf
The keV level (spin f) and the 618 keV level (spin %) have been.interpreted as
15 March 1966
-% 600
700
800
900
Er
1000
the intensity the delayed to the intensity the prompt as function gamma energy. The calculated ia based the assumption the prompt radiation is emitted from the 686 keV level and the delayed radiation from the 618 keV level.
43 and its first rotational state (with a large negative decoupling factor, a = - 1.1). With this interpretation the hindrance of the 618 keV transition is explained by the fact that the Ml transition is K-forbidden. Not so easy to explain is the enhancement of the 625 keV E 2 transition which is not allowed by the asymptotic selection rules. We would like to thank the beta group in Aarhus for providing us with the source material.
lo1 T,=
References 1. K. M. Bisgaard, L. I.Nielsen, E.Stabell and P. Qstergaard, Nuclear Phys. 71 (1965) 192.
2. C. I. Gallagher Jr., W.F .Edwards and G. Manning, Nuclear Phys. 19 (1966) 18. 3. H.A.Vartapetian, Nuclear Phys.32 (1962) 98. 4. H.Langhoff, Phys.Rev.135 (1964) Bl. 5. Yu. K. Shubny, D. K.Kaipov and R. B. Begzhanov, 2h.Eksperim.i Teor.Fiz.47 (1964) 16.
Fig. 1. Time spectrum of &25 keV-Y700 keV coincidences .
528
Volume 20, number 5
15 March 1966
LETTERS
@lb) L’essentiel de la correction due a T(l) aux On a preserve dans l’ecriture de (llb) la reciprocite. grands angles de diffusion peut alors 6tre obtenu en considerant une diffusion Blastique ri petit angle sur le deuton suivi d’un “pick-up” et vice-versa. Des calculs sont actuellement en tours pour &valuer les corrections dues a T(l). R&f&wnces
1. G.Takeda et K.M.Watson, Phys.Rev.97 (1955) 1336.
2. H.Kottler et K.L.Kowalski, Phys.Rev.138 (1965) B619. 3. P. Benoist-Gueutal et F. Gomez-Gimeno, J. Phys. 26 (1965) 403.
4. 5. 6. 7.
K.L.Kowalski, Nuovo Cimento 30 (1963) 266. A.Everett, Phys.Rev.126 (1962) 831. N.M.Queen, Nuclear Phys.55 (1964) 177. P.F.Koehler, E.H.Thorndike et A.H.Cromer,
Phys.Rev.134
(1964) 1030.
*****
THE
LIFETIME
OF SOME
K. PETERSEN, G. TRUWY Laboratoy,
of Applied
Physics
LEVELS
IN 187Re
and P. @STERG~
II, Technical
University
of Denmark
Received 18 February 1966 Using the delayed coincidence method the lifetime of the 686 keV and 618 keV levels has been measured. The lifetime of the 625 keV level is discussed.
The level structure of 187Re has been investigated by many authors. For references, see Bisgaard et al. [l]. As 187Re is situated in the transition region of the deformed nuclei one may expect deviations from the strong coupling scheme. Lifetime determinations of the excited states in 187Re are thus of interest, especially of the 686 keV level, which according to Gallagher et al. [2] might be the K = G - 2 gammavibrational level built on the 206 keV level. As the lifetime measurements [3-51 on the 686 keV level do not agree we have tried to measure the lifetime with the delayed coincidence method. We have used a normal fast-slow system, which for the prompt coincidences in 60~0 gives a resolving time of 0.8 ns and a slope of 0.14 ns. One of the counters detected the beta particles in the energy region 200 - 250 keV and the other counter detected gamma radiation with an energy of about 700 keV using a window of 60 keV. The detectors were NE 102 A plastic scintillators mounted on Philips 56 AVP photomultiplier tubes. In the gamma detector the scintillator was 25 mm thick and in the beta detector the scintillator was 2 mm thick. The source was prepared by evaporation of
Table 1 Lifetime of the 686 keV level 0.29 10.10 8.5 i0.7 0.41 * 0.20 CO.2
Vartapetian [3] Langhoff [4] Shubny et al. [5] This work
ns pa ns ns
natural tungsten on aluminium foil, and the neutron irradiation was performed in DR 2 at Risd. The time spectrum, shown in fig. 1, is complex. It places an upper limit on the lifetime of the 686 keV level of 0.2 ns. In table 1 this result is compared with the earlier measurements [3-51. Table 2 Transition probabilities for the 618 keV and 625 keV levels. The experimental results Tern are compared with the Weisskopf estimates TWeiss T exp
618 keV (Ml+E2) 625 keV (E2) *
Texp /TWeiss
(1.3 hO.1) x 10’ s-l 8.2 xlO1l
s-l
1.9 x 10’4 1.1 x 102
* Using Langhoffs [4] value. See text.
527
Volume 20, number 5
PHYSICS
LETTERS
A component with a lifetime of 0.76 ns is also present in the time spectrum. (This may explain the lifetime measured by Vartapetian [3].) We have measured the ratio of the intensity of this component to the intensity of the prompt coincidences (from l5 the 686 keV level) as function of the gamma energy - and compared it with a calculated curve based upon the assumption that the lifetime must be ascribed to the 618 keV level (fig. 2). Although the uncertainties on lo both curves are great (25%), a comparison of the two curves shows that the lifetime of 0.76 ns belongs to the 618 keV transition and not to the 625 keV transition as the intensity of this is + of the intensity of the 618 keV transition. If we compare this result with Langhoff’s [4] measurements on the level width of the 618 keV + 625 keV transitions 500 (14 r. 618 keV + ro 625 keV = 5.5 x 1O-4 eV) 4oo we get a lifetime of 1.2 ps for the 625 keV level. In table 2 are aiven the measured Fig. 2. Ratio (A) transition probabilities and these are comwith the
Weisskopf
The keV level (spin f) and the 618 keV level (spin %) have been.interpreted as
15 March 1966
-% 600
700
800
900
Er
1000
the intensity the delayed to the intensity the prompt as function gamma energy. The calculated ia based the assumption the prompt radiation is emitted from the 686 keV level and the delayed radiation from the 618 keV level.
43 and its first rotational state (with a large negative decoupling factor, a = - 1.1). With this interpretation the hindrance of the 618 keV transition is explained by the fact that the Ml transition is K-forbidden. Not so easy to explain is the enhancement of the 625 keV E 2 transition which is not allowed by the asymptotic selection rules. We would like to thank the beta group in Aarhus for providing us with the source material.
lo1 T,=
References 1. K. M. Bisgaard, L. I.Nielsen, E.Stabell and P. Qstergaard, Nuclear Phys. 71 (1965) 192.
2. C. I. Gallagher Jr., W.F .Edwards and G. Manning, Nuclear Phys. 19 (1966) 18. 3. H.A.Vartapetian, Nuclear Phys.32 (1962) 98. 4. H.Langhoff, Phys.Rev.135 (1964) Bl. 5. Yu. K. Shubny, D. K.Kaipov and R. B. Begzhanov, 2h.Eksperim.i Teor.Fiz.47 (1964) 16.
Fig. 1. Time spectrum of &25 keV-Y700 keV coincidences .
528