A new N = Z isotope: Krypton 72

Volume 44B, number 5

PHYSICS LETTERS

28 May 1973

A N E W N = Z ISOTOPE: K R Y P T O N 72

H. SCHMEING, J.C. HARDY, R.L. GRAHAM, J.S. GEIGER, Atomic Energy o f Canada Limited, Physics Division, Chalk River Nuclear Laboratories, Chalk River, Ontario, Canada KOJ-1JO

and K.P. JACKSON Physics Department, University o f Toronto, Toronto, Ontario. Canada M5S-1A 7

Received 18 April 1973 Beta-delayed "/-rayshave been observed from the decay of 72Kr (rl/2 = 16.7 + 0.6 s). A decay scheme is proposed based on "/-7 and ~+-7 coincidence measurements. The total decay energy was measured to be QEC = 5057 + 135 keV. The value is compared with mass predictions.

The investigation of mirror and self-conjugate nuclei heavier than A = 56 not only provides a potential new field for studying isospin breaking effects, but also serves as an excellent testing ground for mass formulas [ 1 - 3 ] , since here predictions of ~-decay energies differ by more than one MeV from one formula to another. This has led us to measure the decay properties of 72Kr, the heaviest self-conjugate nucleus yet observed. The new isotope was produced in the 58Ni(160, 2n)72Kr reaction with the 160 beam from the upgraded Chalk River MP tandem. The beam entered a helium filled target cell through a tantalum window and bombarded the target at 55 MeV incident energy. The target was a self-supporting 2.7 mg/cm 2 thick 58Ni foil, enriched to 99.9%. Reaction products recoiling out of the target were thermalized in the helium which was periodically swept to a shielded 200 cm 3 counting cell. En route, the carrier gas passed through a glass-wool particulate filter, a cold trap (-130°C) and a titanium getter oven which together removed most of the unwanted components. At a predetermined time during each counting period the counting cell was pumped out. This removed - and thus identified - volatile compounds leaving nonvolatile compounds stuck to the walls of the cell. The counting period was divided into eight equal intervals, the data from each being routed into separate spectra. Gamma-ray singles spectra were measured with a 50 cm 3 Ge(Li) counter while for 3'-7 coinci-

dence measurements an additional 10 cm X 12.5 cm NaI(T1) detector was used. The fl+-3' coincidence measurements made use of a counting cell machined from NE 102 plastic scintillator that was attached to a Philips XP 1040 photomultiplier. The walls of the cell were 2.5 cm thick, sufficient to stop and detect positrons from the decay of 72Kr. All coincidence data were recorded event by event on magnetic tape. Fig. la shows parts of the ~/-ray spectrum recorded before and after the counting cell had been pumped out. The bars indicate the height that would be expected for the peaks if they originated from non-volatile compounds. We find peaks from volatile compounds at 124.3,162.6, 178.1,252.2,310.0, 415.0, 511.0 and 576.6 keV. The decay constants measured for these lines are given in table 1 where the numbers shown are the results of several runs with different time sequences. The main activities remaining after the counting cell had been pumped out corresponded to the strongest transitions in the decays of 71As, 72As, 71Se and 72Br and established that all of these elements appear in non-volatile compounds. Positrons spectra were obtained in coincidence with all strong ~,-ray lines in the spectrum. We determined the response function of the system in situ using positrons from the decay of 35Ar. This decay is well suited for calibration, since it has a single strong/3 +. transition [4] of 4941.6-+ 1.7 keV, slightly above the end point expected for 72Kr. We produced 35Ar before and after measuring 72Kr, first via 449

Volume 44B, number 5

PHYSICS LETTERS

28 May 1973

b

310.0 keV

I

8000

IOC 415.0 keY

J 4000 W

rz Br

\

Z Z

\

\ I0 0

5

IO

15

tl

20

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---t/

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,

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,

;

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,

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320 410 ENERGY (keV)

I

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1

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,

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Fig. 1. (a) Parts of the ',/-ray spectra: the upper spectrum was counted for 28 s after filling the cell; the lower spectrum was counted for the 4 s subsequent to evacuation. (b) The a-spectrum in coincidence with the 415.0 keVT-tramition from 72Kr. (c) Growth and decay of the 862.0 keV peak from 72Br. 2°Ne(160, n)35Ar and then via 35C1(p, n)35Ar. An additional check was the/3 + spectrum of 72As, a contaminant always present in our ~ - 7 coincidence runs. We scaled the measured 35Ar/3-spectrum to fit each unknown spectrum and determined the end-point energy from the scaling factor of the energy coordinate.

The results are shown in table 1. The procedure was verified by measuring the end-point energy of 72As (energy scaling factor of ~ 0.4 compared with ~. 0.7 for 72Kr) which agreed well with its known value [4] ; see table 2. Fig. lb shows as an example the positron spectrum in coincidence with the 415.0 keV "r-ray;

Table 1 Experimental results

72Kr:

73Kr:

7-ray energy* (keV)

decay constant ~. x 104 (s - l )

relative intensities

/3+ end-point

124.3 162.6 252.2 310.0 415.0 511.0 576.6

347 492 335 415 422

19 40 17 77 100 1040 33

3431 3794 3329 3626 3682

178.1

315 ± 65

*All values ± 0.3 keV.

450

± 67 ± 50 :~ 113 ± 31 ± 21

393 ± 40

±3 +4 ±2 +5

energy (keV) ± ± ± ± ±

210 180 500 105 80

± 120 ±3

3364 ± 155

-

5589 ± 350

PHYSICS LETTERS

Volume 44B, number 5

28 May 1973

Table 2 Total decay energies (QEc) decay 72Kr~ 72Br 7aKr ~ 7aBr 71Seo 71As 72As---~-72Ge

present measurements (keV) 5057 6789 4428 4234

-+ 135 ± 440* ± 125"* ± 140

previous measurement (keV)

predicted QEC (keV)

4351 -+ 7 [4]

ref.[1]

ref.[2]

ref.[3]

6526 8027 5281 4416

5140 6960 4580 4110

5067 7065 4514 4213

*Calculated by assuming that the 178.1 keV ")'-rayfeeds the ground state of 7aBr. **Decay scheme from ref. [8]. the solid line is the scaled down 35Ar spectrum shape used in its analysis. We attribute all lines listed in table 1 to 72Kr, except for the line at 178.1 keV, on the basis that: a) they all stem from a volatile compound (i.e., they cannot be from Br, Se or As); b) their decay constants all agree within errors; c) the average decay constant is compatible with the growth of 72Br, the daughter of 72Kr; d) their/3 + end-point energies are all consistent with one another; and e) none of the lines was produced in a control run with a 60Ni target at the same incident energy. We determine the half-life of 72Kr to be 16.7 + 0.6s (cf. ref. [5]). Fig. lc shows growth and decay o f the 862.0 keV line, the strongest line from the decay [6] of 72Br. The solid curve is calculated from the measuredlifetime for 72Kr and rl/2 = 72 s for 72Br; it fits the data well. We observed three other lines which possibly originate from 72Kr: weak peaks at 438 -+ 2 keV and 559 + 2 keV, and a peak at 147 +- 1 keV masked by a 71Se impurity. No further indication of lines from 72Kr has been found in the range from 50 keV to 2.5 MeV. We attribute the activity at 178.1 keV tentatively to 73Kr, despite disagreement between our measured half-life of 22 + 4 s and the previously reported value [7] of 34 + 4 s, since it originates from a volatile compound, it has a higher ~ end-point energy than 72Kr, and it was also produced at 65 MeV incident energy on 60Ni, presumably via 60Ni (160, 3n). In table 1 we give intensities for ")'-rays from 72Kr including the annihilation radiation. Special precautions were taken to determine the latter by subtract-

ing contaminant activities and correcting for annihilation in flight. The ")'-~ coincidence measurements show that the 310.0 keV and the 415.0 keV transitions in 72Br are not in cascade. On this basis and supported by energy sums we propose the 72Kr decay scheme shown in fig. 2. It accounts for all but one of the 72Kr 3,-rays from table 1 as well as the 147 keV line. The/3"- end-point energy corresponding to the 124 keV peak indicates that the ")'-transition must be in cascade with one of the other strong lines, but there is insufficient 3'-~' coincidence data to establish which 0+

5057

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+ LOG f~~

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162.71r-- <4 %

>5.5

72Br Fig. 2. Proposed decay scheme for 72Kr. Quoted branchk~g ratios include the effects of electron conversion (for assumed MI transitions) and K-capture. See comment in text on 1÷ spin assignments. 45J

Volume 44B, number 5

PHYSICS LETTERS

one. Thus, another 4%/3-branch must exist whose intensity would be subtracted from one of the branches shown to excited states in 72Br; this would not alter the 54% ground-state b r a n c h t . The 1÷ spin assignments follow from selection rules for allowed ~3-decay if one assumes that the decay originates from the 0 ÷, T = 0 ground state of 72Kr. It should be noted, however, that any one o f these states could be 0 + if the 72Kr ground state had a T = 1 admixture o f a few percent. F r o m this decay scheme and the end-point measurements listed in table 1 we establish the total decay energy for 72Kr shown in table 2; the q u o t e d e r r o r includes the uncertainty in placement of the 124 keV 7-transition. Also listed in the table are our results for 73Kr, 71Se and 72As. Our measurements agree well with the predictions of Zeldes et al. [3] and Garvey et al. [2] but disagree with those of Myers and SwiatWe have assumed the lowest 1+ state we observe to be the ground state. Very recent work (J.H. Harililton, private communication) on the decay of 72Br indicates a ground state assignment of or = 3. To be consistent with our observations, this would require the ground state to lie < 60 keV below our lowest state. The change in QEC is thus unlikely to be outside the quoted error.

452

28 May 1973

tecki [ 1 ]. A systematic investigation of masses in this region is clearly desirable; we anticipate that the techniques described here will provide a selective and powerful m e t h o d for doing so.

References [1] W.D. Myers and W.J. Swiatecki, Report UCRL-11980 (1965). [2] G.T. Garvey et al., Revs. Mod. Phys. 41 (1969) S1. [3] N. Zeldes, A. Grill and A. Simievic, Mat. Fys. Skr. Dan. Vid. Selsk. 3, no. 5 (1967). [4] A.H. Wapstra and N.B. Gore, Nucl. Data Tables 9 (1971) 265. [5] After completion of this work C.N. Davids and D.R. Goosman (private communication) repeated part of the experiment in Brookhaven and confirmed our identification and half-life for 72Kr. [6] R.L. Robinson et al., Bull. Amer, Phys. Soe, 16 (1971) 626; J.H, Hamilton, private communication. [7] P. Hornsh~j, K. Wllsky, P.G. Hansen and B. Jonson, Nucl. Phys. A187 (1972) 637. [8] U. Hundelshausen, Z. Phys. 225 (1969) 125.