Decay of the mirror nuclide 45V

Volume 116B, number 1

PHYSICS LETTERS

30 September 1982

DECAY OF THE MIRROR NUCLIDE 45V P. HORNSHOJ, J. KOLIND and N. RUD Institute of Physics, University of Aarhus, DK-8000 Aarhus C, Denmark

Received 8 June 1982

The 9+ decayof 4Sv (jr, T = 7/2-, 1/2) has been observed. The half-life was found to be 539 +-18 ms; in addition to the superallowed transition to the mirror state (4STi ground state), it exhibits a (4.3 -+1.5)% allowed branch to the 5/2state at 40.1 keV in 4STi. Decay data for the complete f7/2 shell series of mirror nuclei are presented.

The nuclide 45V is a member of the series of eight T z = - 1 / 2 nuclei in the f7/2 shell, which decay pre-

dominantly by superallowed/3-decay to the ground state of their mirrors ( T z = 1/2). Since the investigations of 47Cr, 51Fe, and 55Ni by Hornsh~j et al. [1] and 49Mn by Hardy et al. [2], only 45V lacked a description in terms of half-life and other decay properties. For mirror decays, these parameters are difficult to measure because of the fast decays with only weak - if any - transitions to excited states in their daughters. In addition, the cross section leading to the nuclide under investigation is often only a small fraction of the total reaction cross section. The mass of 45V has already been determined [3]. The present work reports the first characteristics of the decay of 45V. The 45V activity was produced in the 4°Ca(7Li, 2n) 45V reaction by bombarding 350-500/~g/cm 2 targets of calcium metal, both natural calcium and calcium enriched to 99.96% 4°Ca with 7Li ions from the Aarhus University EN tandem accelerator. The optimum conditions, deduced from the production yield of 45V in both 6Li and 7Li bombardments with lithium ions in the energy range 1 0 - 1 9 MeV, were established at 14 MeV 7Li bombarding energy. In the analysis, the 56.6 keV "r-ray assigned to 45V by Gronemeyer et al. [4] was used as a measure of the yield of 45V. In subsequent experiments, the 45V activity was transported to the measuring stations by a heliumjet transport system (HeJTS). The 45V recoils were stopped in the target chamber in 1 atm. helium gas 4

loaded with AgC1 aerosol and transported to the skim. mer catcher chambers, where the 45V nuclei were deposited on aluminum catcher foils in front of Ge(Li) X-ray or T-ray detectors. With the system operating in a continuous mode, a 40.1 keV 7-ray agreeing very closely with the energy of the second excited state of 45Ti [5] was observed, indicating that a measurable fraction of the 45V decays proceeds via this level. The exact location of the 40.1 keV 7-ray of 45 Ti is established from the in-beam spectrum, where 45 Ti ")-rays from the 40Ca(7 Li, np) 45 Ti reaction are strong. Comparisons with neighbouring nuclides indicate that branchings to excited states are expected to be small, and within the accuracy of present experiments, no other 7-rays which could be assigned to 45 Ti populated in the decay of 45 V, were observed. The assignment is backed by an experiment, in which all operating parameters of the system were left unchanged, with the exception that the HeJTS was stopped by evacuation of the target chamber. In this experiment, no trace of the 40.1 keV 'r-ray could be observed. With the aid of this information, the half-life of 45V was measured, using a slow beam-pulsing system. Activation and counting intervals in most of the experiments were 1.0 s and 3.0 s, respectively. All the ')'-rays measured at the catcher position were tagged with their time of arrival, measured relative to the termination of activation. This two-dimensional distribution of events was recorded on magnetic tape for later off-line analysis. The catcher foils were renewed at reg0 031-9163/82/0000-0000/$02.75 © 1982 North-Holland

Volume 116B, number 1

PHYSICS LETTERS

30 September 1982 I

I

t

7126

Ag K-X

7/2-

9000 N y (40.1 keV) 46ms( 3(142.5

45v y401

-J 1C <>

,~ 6000

,

,,,=

z uo

÷

~*/o 40.1

+{

tog It

5/~1 4.3 4.98 363

49Cr "(905

45Ti

52Ti ~(124.4

o

o 300C

2

~T1/2

I

= 542±25 ms

Z o

01

I

25

so

7'5

ido Ey (keY)

~s

102

16o

Fig. 1. Spectrum of v-rays recorded with the 1 cm3 Ge(Li) X-ray detector at the catcher position of the HeJTS. In addition to the 40.1 keV v-ray, several -r-rays from activities produced in the target (and the entrance window) are labelled with their origin. Also observed are Ag K X-rays from fluorescence in the AgC1deposit at the catcher foil. ular intervals to remove long-lived activities. The resulting ?-ray spectrum recorded in the first 0.75 s of the counting intervals is shown in fig. 1. A very prominent 40.1 keV 7 line is seen together with identifiable contaminant lines. The result of one of the half-life experiments is shown in fig. 2, where the decay of 45V is represented by the decaying intensity of the 40.1 keV transition in 45 TL The 511 keV annihilation radiation could not be used in the half-life determinations due to a strong admixture of 42Sc activity with a half-life of 0.68 s. The analysis of the available data yielded the result Tl/2(45V) = 539 -+ 18ms. As the ?-decay branch to the 40.1 keV level of 45Ti cannot be determined via a comparison with the annihilation radiation without access to mass separation, an alternative method was employed. This relies on the fact that the 7-ray in 45V, which is emitted between the same isobaric analogue states as the 40.1 keV v-ray of 45 Ti has been identified by Gronemeyer et al. [4] as a 56.6 keV transition. In experiments, where the activation and counting periods were 1.0 s each, the lowenergy 7-ray spectrum was recorded directly at the tar-

1(

0

I

I

1

2

3 SECOND

I

I

4

5

Fig. 2. Decay curve observed for the 40.1 keV 3,-ray. In the fitting procedure, the first 0.5 s period has been omitted to exclude effects from the finite average transfer time (about 170 ms, as measured with 13-delayede-particles from ~°Na, produced in the 19F(3He, 2n) reaction). The inset shows the decay scheme proposed on the basis of the available data. get position with the Ge(Li) X-ray detector, the events from the two periods being routed to two separate spectra. From these spectra, with knowledge of the 45V half-life and the detector efficiency, the yield of 40.1 keV 7-rays, representing the/~+ branch to the 40.1 keV level of 45 Ti, may be related to the intensity of the 56.6 keV "/-ray, representing the production of 45V. Both transitions are assigned M1 multipolarity [4], and including the contribution from internal conversion, the intensity ratio I(40.1)/I(56.6) of the two transitions yields (6.2 -+0.9)%. This intensity ratio may be converted into a branching ratio provided the fraction o f 45V nuclei emitting a 56.6 keV 7-ray during de-excitation is known. This fraction has not been determined experimentally, and the present analysis thus had to rely on a theoretical calculation o f the distribution of 45V nuclei into the various final levels after neutron emission, combined with experimental knowl-

Volume 116B, number 1

30 September 1982

PHYSICS LETTERS

Table 1 Superallowed/3+ decays in the f7/2 shell. Nucleus

jrr

T1/2 (ms) a)

I#(%) b)

Q (keV)

log ft

Ref.

418c

7/27/27/23/25/25/27/27/2-

596 (2) 513 (8) 539(18) 452 (18) 384 (17) 245 (7) 265 (25) 189 (5)

100 (100) 95.7 >92 93.6 >96 (100) 100

6495 (2) 6866 (8) 7128 (17) 7440 (12) 7716 (24) 8039(12) 8291 (18) 8691 (10)

3.46 3.51 3.64 3.64 3.67 3.54 3.63 3.59

[9] [3,11] [3,present] [1,3] [ 2,3] [1,3] [ 3,10] [1,3]

43Ti 4SV 47Cr 49Mn SlFe S3Co

SSNi

a) The values in brackets represent the uncertainties in units of the last digit. b) Where lower limits are given, a 100% branch has been assumed in the caleulation of the log ft value. For 41Sc and SSNi, the first-excited state in the daughter is located around 2MeV; thus 100% g.s. decay seems very probable. Corrections for electron capture were taken from ref. [12].

edge of the de-excitation pattern of the 45Vmirror [6] in the lower excitation-energy range. The calculations were performed with a version of the nuclear-evaporation code GROGI2 [7], modified to take into account the experimentally known discrete levels close to the yrast line in the residual nuclides in the various steps of the reaction. The protonbinding energy of 45V is only about 1.6 MeV, and all bound states are probably known, as can be inferred from the much better known level structure of 45 Ti. Only some high-spin states above 1.6 MeV may decay by 7-emission and contribute to the 45V cross section. The high-spin negative parity f7/2 states are known from the isobaric analogue nucleus 45 Ti to decay mainly to the 7 / 2 - ground state. A positive-parity deformed band has been identified in 45Ti (and 45V), and the decay of this band is known both in 45 Ti and 45V. The calculations show that if all possible uncertainties in the decay paths of the excited states in 45V are taken into account (70 -+ 20)% of all 45V nuclei should pass through the 56.6 keV level in the de-excitation process. Since only knowledge of relative feeding to the levels is required, this result is essentially independent of the Li beam energy and does not vary with even large variations of the parameters entering into the statistical calculation. Based on this information, the production rate of 45V is determined, and the 13+ branch to the 40.1 keV level in 45 Ti is found to be (4.3 +- 1.5)%, a weak branch consistent with the systematics of other f7/2 mirror decays. A summary of the results is shown in the in-

set of fig. 2. The log ft values are calculated with the f-values from ref. [8]. The log ft value for the transition to the 40.1 keV, 5 / 2 - level indicates an allowed transition. The result for the superallowed transition to the 45 Ti ground state is given in table 1, where the data for the complete f7/2-shell series of mirror nuclei are presented.

References [1] P. HornshCj, L. HCjsholt-Poulsen and N. Rud, Nucl. Phys. A288 (1977) 429. [2] J.C. Hardy et al., Phys. Lett. 91B (1980) 207. [3] D. Mueller, E. Kashy, W. Benenson and H. Narm, Phys. Rev. C12 (1975) 51. [4] S.A. Gronemeyer, L. Mayer-Schtitzmeister and A.J. Elwyn, Phys. Rev. C21 (1980) 1290. [5 ] J.H. Jett, G.D. Jones and R.A. Ristinen, Phys. Left. 28B (1968) 11. [6] K.V.K. Iyengar, E. Wong and G.C. Neilson, Nucl. Phys. A155 (1970) 582. [7] J. Gilat, GROGI2 - A nuclear evaporation computer code, report BNL-50246 (1970). [8] D.H. Wilkinson and B.E.F. Macefield, Nucl. Phys. A232 (1974) 58. [91 D.E. Alburger and D.H. Wilkinson, Phys. Rev. C8 (1973) 657. [10] S. Kochan, B. Rosner, I. Tserruya and R. Kalish, Nucl. Phys. A204 (1973) 185. [11] P.M. Endt and C. van der Leun, Nucl. Phys. A310 (1978) i. [121 N.B. Gore and M.J. Martin, Nucl. Data Tables 10 (1971) 205.