Volume 58B, number 3 PHYSICS LETTERS 15 September 1975

IN-BEAM a- AND 7 -RAY SPECTROSCOPY FOR 216R,

T. NOMURA’, K. HIRUTAZ, M. YOSHIE3, H. IKEZOE4,

T. FUKUDA4 and 0. HASHIMOTOS

Cyclotron Laboratory, The Institute of Physical and Chemical Research, Wako-shi, Saitama, 351, Japan

Received 15 July 1975

By a simultaneous observation of *particle decays and T-rays in the 208Pb(r2C, 4n) and 207Pb(r2C, 3n) reac- tions, we have found extremely short-lived long-range or-particle decays from high-spin states in 216Ra and estab- lished their decay properties. The level sequence of ‘raRa was established up to 11. (possibly 14+).

Many neutron-deficient translead nuclei are char- acterized by having large &decay Q-values (Q,). and consequently short lifetimes ranging from seconds to submicroseconds. Since excited states of such nuclei sre expected to have extremely short a-decay half- lives, the competition between o-particle and r-ray emission can be significant. For instance, the ground state of 216Ra 128 is known to decay with Q, = 9559 + 8 keV and tl12 =182+10ns [l].Thus,theQ,of a 1.5 MeV level in 216Ra would amount to 11 MeV and its partial a-decay half-life could be around 100 ps if the hindrance factor for the long-range o-particle decay is assumed to be the same as for the ground- state decay. This decay time is of same order of mag- nitude as expected for a 500 keV E2 r-transition in this nuclear region. In-beam o-ray combined with -y- ray spectroscopy is considered, therefore, a powerful method for the investigation of such short-lived (Y- emitters. Motivated by the above consideration we have carried out in-beam (Y- and r_ray spectroscopy for 216Ra whose level structure was completely un- known prior to this work. Because the usual helium- jet transport technique [2,3] is not applicable to the short o-decays of interest, the present experiment was performed using the pulsed-beam method successfully applied previously [4].

’ Present address: Dgpartement de Physique Nucdaire, C.E.N. Saclay, BP 2,PllP0 Gif-sur-Yvette, France. .

2 Present address: Tokyo Gakugei University, Tokyo, Japan. 3 Present address: The Institute for Nuclear Study, Tokyo,

Japan 4 On leave of absence from Tokyo University of Education,

Tokyo, Japan. 5 On leave of absence from Tokyo University, Tokyo, Japan.

216Ra was excited in the 208Pb(12C, 4n) and 207Eb(12C, 3n) reactions by bombardment of self-

supporting targets (enriched to 98%) with 60-90 MeV 12C ions accelerated from a cyclotron at the In- stitute of Physical and Chemical Research. The cy-par- titles and Frays were usually detected with 200 E.tm Si and 40 cm3 Ge(Li) detectors, respectively. Time dis- tributions of OL- and -y-rays were taken by use of the natural beam bursts of the cyclotron having a width of around 2 ns and a repetition period of 130 ns at 80 MeV 12C ions. The efficiencies of both counters were calibrated by using (Y- and r-ray standard sources with known intensities, and excitation functions, an- gular and time distributions were measured for cu-parti- cles and y-rays simultaneously, so that relative inten- sities of (Y- and prays could be compared.

Fig. l(a) shows a partial delayed o-particle spec- trum resulting from the 208Pb t 12C reaction at 79

MeV for a 1 mg/cm2 thick target. Three prominent peaks at 8698,902l and 9349 keV originate from the ground-state o-decays of 215Ra, 217Ra and 216Ra produced in the (12C, Sn), (12C, 3n) and (12C, 4n) reactions, respectively [l]. Four peaks at 10491, 10823,11028 and 11345 1eV were unambiguously assigned, based on their excitation functions, to long. range a-particle decays from excited states of 216Ra. Decay curves of these peaks are shown in fig. l(b). It is clear that the first three transitions decay with the same half-lives of about 7 ns, indicating the existence of an isomeric state with this half-life. For the 11345 keV peak no definite half-life could be obtained be- cause of its weak intensity. Angular distributions of these four peaks were found to be strongly anisotrop-

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Volume 58B, number 3 PHYSICS LETTERS 15 September 1975

12K

3K

0

(a)

. 9.4 9.6 9.8 10.7 10.9 lit

Ed MeV >

lo1 L L

b)

80

Fig. 1. (a) Part of a delayed or-particle spectrum from the 208Pb + 12C reaction taken with the Si detector placed at 135” to the beam direction (b) Time distributions of the long-range or-particle decays

ic, so that the angular momenta carried by these tran- sitions were definitely non-zero. Besides the above transitions, another rather strong peak came up at

9551 keV when we used a thinner (about 400 pg/ cm2> target in the same reaction to obtain better en- ergy resolution (see the inset of fig. l(a)). This peak was also assigned to 216Ra from its excitation func- tion although neither the angular distribution nor the lifetime was measured. Intensities of the long-range a-particle decays relative to the ground-state decay were estimated by considering their angular distribu- tions, except for the 955 1 keV peak whose intensity was evaluated at 135”.

Results of the y-ray measurement are summarized in table 1. The 343.5 keV and 343.3 keV -r-rays ap- peared as a single peak in the Ge(Li) distribution men- tioned before, but could be rather well resolved by use of an intrinsic Ge detector with a resolution of

274

0.8 keV at this energy. The first four y-rays listed in table 1 were found to decay with a half-life of about 7 ns, and their angular distributions are those expected from a stretched sequence of quadrupole transitions. When we consider total internal conversion coeffi- cients (ICC) calculated from available tables [5,6], the yields of these four y-rays are consistent with the assignment that they are cascade E2 transitions fol- lowing the 7 ns isomer and reaching the ground state. It should be noted here that the intensities of these y-rays relative to the ground-state a-particle decay are useful to exclude the possibility of M2 assignments, since the ICC are quite different for electric and mag- netic transitions at energies of interest. In fact, an M2 assignment for the above y-rays would result in the fact that the yield for each transition is larger than that of the ground-state o-particle decay. The order- ing of these transitions was determined by their exci-

Volume 58B, number 3 PHYSICS LETTERS 15 September 1975

Table 1 Gamma-ray transitions in 216Ra observed from the 20*Pb + r2C and ‘O’Pb + 12C reactions.

Er (keW Ji+Jf 208Pb + 12C (79 MeV) Conversion c) 207Pb + 12C (63 MeV)

A2 a) zr (total) b) Zr (delayed) coeff. (Cal)

z (total) d)

687.9 2+ + o+ 0.21 [ll 111 0.019

(E2) 0.211 (M2) [ll

475.5 4 + 2+ 0.25 1.02 (7) 0.94 0.043

(E2) 0.61

0.92

343.5 6+ +4+ 0.33 0.93 (7) 0.99 0.10

(E2) 1.69 (M2)

0.79

203.5 8+ +6+ 0.27 0.69 (5) 0.67 0.53

(E2) 9.44 (M2)

0.73

314.6 10+-t 8+ 0.21 0.72 (5) -0 0.13

(E2) 2.23 (M2)

0.30

309.1 11-j 10+ -0.26 0.76 (5) -0 0.033

(El) 0.67 (Ml)

0.19

612.9 (12++ 113 -0.09 0.56 (6) -0 0.008

(El) 0.108 (Ml)

< 0.05

344.3 (14++ 123 0.28 0.50 (7) -0 0.10

(E2) 1.67 (M2)

< 0.05

Ground-state or-decay 0.95 (6)

a) Angular distribution coefficients for the Ps (cos 0) term. b) Relative -y-ray intensities estimated by averaging their angular distributions. Errors are given in brackets. c) Calculated from refs. [5] and [6]. d) Relative intensities corrected by conversion coefficients, for which the upper value of each row is used.

tation functions measured in the 207Pb (12C, 3n) reac-

tion in the vicinity of the Coulomb barrier, i.e., from 60 to 68 MeV, where the maximum angular angular momentum brought into the system is relatively small and therefore a large difference is expected in their relative intensities. The multipolarities and ordering of other y-rays were determined in the same way. The assignment for the 612.9 and 344.3 keV -y-rays is, however, somewhat ambiguous, especially for the or- dering of these two transitions.

Fig. 2 shows a decay scheme of 216Ra established in the present work. Energies and J” values were de- termined mainly from the results of the r-ray spec- troscopy. Based on energy relations, lifetime measure- ments and angular distributions, the long-range cw-par- title transitions observed could be placed into this level scheme very well. The ratios of the a-particle de- cay width F, to the total width Ftotal (= Fa + Fr) were evaluated from the measured branching ratios of (Y- to r-transitions and calculated conversion coefficients

[5,6]. It should be mentioned that o-decays of 8+ +

8+ and 8+ + 6+ must exist because they are more fa- vored, due to the smaller spin difference, than the observed 8+ + 4+ transition. These possible o-particle transitions were unfortunately masked by the strong

ground-to-ground decay because their energies are nearly the same.

In conclusion, we believe that the present work has proved the usefulness of in-beam cr- and ‘y-ray spec- troscopy for nuclei having very short a-decay half- lives. The present method also makes it possible to measure angular distributions for long-range a-particle decays, which have turned out to be strongly aniso- tropic for 216Ra. Because of the large spin difference between initial and final states, angular distributions must be affected appreciably by statistical tensors of high degree like &j and p8 which can hardly deter- mined by y-ray spectroscopy.

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Volume 58B, number 3 PHYSICS LETTERS 15 September 1975

-t--- (14+)(3291.3)

-c""- (W)(29470)

\ 61 2.9 I

11- 2334.1

lo+ 20250

8+ 1710.4

6' 1506.9

4+ 1163.4

2+ 687.9

Fig. 2. A decay scheme of 216Ra established in this work. Note that cc-particle decays are expressed in Qo instead of Eo shown in fig. 1, where the conversion was made by taking into account the recoil energies and screening effects of atomic electrons. As noted in the text, the assignment for the 12+ and 14+ states is tentative. The level scheme of 212Rn is taken from ref. [ 71.

References

[l] T. Nomura et al., Nucl. Phys. A219 (1973) 253. [2] K. Valli, W. Treytl and E.K. Hyde, Phys. Rev. 161 (1967)

1284. [3] D.F. Torgerson and R.D. McFarlane, Phys. Rev. C2

(1970) 2309. [4] T. Nomura and K. Hiruta, Nucl. Instr. 108 (1973) 61. [S] R.S. Hager and E.C. Seltzer, Nucl. Data Tables A4 (1968)

1. [6] 0. Dragoun, Z. Plajner and F. Schmutzler, NucL Data

Tables A9 (1971) 119. [7] K.H. Maier et al., J. de Phys. C6 (1971) 221.

-tf o+ 0

“‘Rn ’

276