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The ft value of the superallowed Fermi decay 46V(β+)46Ti
Volume 65B, number 2
PttYSICS LETTERS
THE ft VALUE OF THE SUPERALLOWED
8 November 1976
F E R M I D E C A Y 46 V(fl+ )46Ti
G.T.A. SQUIER and W.E. BURCHAM Physics Department, University of Birmingham, UK S.D. HOATH* Nudear Physics Laboratory, Oxford, UK and J.M. FREEMAN, P.H. BARKER** and R.J. PETTYt Nuclear Physics Division, AERE, Harwell, Oxon, UK Received 15 September 1976 The 46Ti(p, n)46V reaction threshold has been measured as 8007.7 + 1.8 keV, equivalent to an energy release of 6030.6 + 1.8 keV in the beta decay 46V(fl+)46Ti. The 46V half-life has been measured as 422.28 + 0.23 ms. The corresponding ft value is reasonably consistent with those of other superallowed Fermi decays.
The conserved vector current hypothesis of basic weak interaction theory predicts that, for all superallowed Fermi decays between j~r = 0 +, T--- 1 analogue states, t h e f t values should be the same after correction for nuclear charge-dependent and electromagnetic radiative effects [1 ]. Within the uncertainties in the theoretical corrections, and the experimental errors in the measurements of the beta end-point energies and half-lives, on which the f t values depend, recently published surveys [ 2 - 4 ] of existing data conclude that the criterion of equality of the f t values appears to be met to within a few tenths percent. However, a close inspection of the present experimental situation in the case of the decay 46V(/3+)46Ti shows that there remain some questions concerning both its beta end-point energy and half-life; these will be discussed in this paper. Considering first the beta end-point energy E O, we note that the first result published for this with a quoted error < 0.05% was that obtained from a measurement at Harwell [5] of the threshold for the 46Ti(p, n)46V reaction. From this threshold the equivalent E 0 was deduced as 6032.1 -+ 2.2 keV. This becomes 6031.4 -+ 2.2 keV using the present value of the 212po c~-particle energy standard [6]. * Now at the Rutherford Laboratory, Chilton, Oxon, UK. ** On leave from the University of Auckland, New Zealand. Now at the Physics Department, University of Melbourne, Australia. 122
More recently Hardy et al. [7] have derived the result 6018.8 -+ 2.8 keV for the 46V end point energy E 0 from their measurement of the Q-value for the reaction 46Ti(r, t)46V relative to that for 27A1(~-, t)27Si. In discussing the large discrepancy with the earlier E 0 value, the authors pointed to the possibility of compoundnuclear resonances having affected the (p, n) threshold measurements on which the earlier result was based. Such effects can indeed occur as we have found, for example, in recent studies of the 54Fe(p, n)54Co threshold [8]. The Chalk River result for 46V has therefore been adopted in the latest compilations of superallowed Fermi decay data [ 2 4 ] , and when combined with the accepted value for the 46V half-life, it has led to anft value showing good consistency with the average for all such decays. However, Hardy has since reported [9] an error of several standard deviations in the Chalk River endpoint value for 46V, attributed to a contaminant peak in the (r, t) measurement. This has been confirmed in a measurement of the same (r, t) reaction Q-value by Gl~/ssel et al. [10]. Their preliminary result corresponds to an E 0 value of 6029 + 2 keV, consistent with the result from the earlier (p, n) threshold measurement. In the meantime we have been making a fresh study of the 46Ti(p, n)46V threshold, applying the tech-
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PHYSICS LETTERS
niques used in the case of 54Fe(p, n)54Co. The procedure followed was that of repeated irradiation of the target (400/ag/cm 2 of 81% 46Ti on a gold backing) by proton beam bursts of duration about a half-life (0.4 s for 46V), the relative reaction yield being followed as a function of proton energy by detecting positrons from 46V in a plastic scintillator, between the beam bursts. Positron pulses were accumulated in a 128channel multiscaler with 0.02 s channel stepping, for several hundred irradiation cycles at each proton energy, and the short-lived component was extracted from the observed decay curve. In this way the 46V yield could be followed down to a level of around 10% that of the background. The method provided a sensitive means of checking the shape of the yield curve in the threshold region, thus allowing the possible presence of resonance effects to be investigated. The yield curve was analysed according to the thick target 3/2 power law over a range of ~7 keV above the energy at which 46V activity was first detected; beyond this range the effect of a resonance was evident. The proton energy scale was calibrated as previously described [8] in terms of the 8784.37 -+ 0.07 keV a-particle group from a 212po source [6]. Seven independent threshold measurements were made, and the mean result, with allowance for discrete energy loss in the target and finite beam energy spread was 8007.7 k 1.8 keV, leading to a - Q value of 7835.2 -+ 1.8 keV, and an end-point energy for the 46V(/3+)46Ti decay of 6030.6 + 1.8 keV. This result is consistent with the earlier Harwell measurement and also with the result reported by the Munich Group [101. The value for the 46V end point from the present experiment, taken in conjunction with the recommended value of 424.4 + 1.4 ms [3, 4] for the halflife, corresponds to an ft value for this superallowed Fermi decay, which is more than two standard deviations higher than the weighted mean o f the other seven most precisely determined ft-values. Very recently Wilkinson and Alburger [11 ] have reported a new measurement of the 46V half-life, giving the result 424.01 + 0.47 ms. With this more precise value the discrepancy between the 46V d e c a y f t value and the mean fTvalue becomes more marked. Following the calculative procedure of ref. [2], these are 3109 + 6 s and 3088 -+ 2 s, respectively. The absolute ft values depend to a small extent on the values chosen
8 November 1976
for the charge-dependent corrections [2, 3, 4] but the discrepancy remains. However, we have also been carrying out an independent study of the 46V half-life, using the 46Ti(p, n)46V reaction on the enriched 46Ti target described above, at a proton energy of 8.15 MeV corresponding to a resonance not far above the reaction threshold. We have paid careful attention to possible sources of systematic error, particularly from contaminant activities and from pulse pile-up effects, using the method of decay curve analysis described by Robinson [12]. The only significant contaminant found was a 4 s activity with ~4 MeV end point, identified as 27Si from the (p, n) reaction on 27A1. This had a discernible effect on the deduced half-life for 46V in some of the runs, and these were analysed by a double-exponential fitting program. The effects of pulse pile-up were deter. mined experimentally, and small corrections were found necessary in some of the runs. Further details of the measurements and analysis will be reported elsewhere. The weighted mean of groups of runs made under a variety of conditions was 422.28 ms, with an internal statistical error of +0.23 ms, which was slightly greater than the external scatter error and is therefore adopted. This result is significantly lower than the other measurement of comparable precision, that of Wilkinson and Alburger [11] quoted above. The results of the present measurements are set out in table 1. The effective f t value for 46V decay is calculated following the procedure of Wilkinson [2], with ft =fR t(1 -- e) where fR is the integrated Fermi function corrected for outer electromagnetic radiative effects up to order Z2c~3 [13], t is the half-life corrected for electron capture, and e is the charge-dependent correction. The calculated ft value for 46V decay is reasonably consistent with those for the other superallowed Fermi decays. Table 1 The ft value for the decay 46V(~+)46Tifromthe present work
46Ti(p, n)46Vthreshold Q value
46V(fl+)46Tiend-point energy half-life
fR a) t e a)
ft
8007.7 -+ 1.8 keV -7835.2 -+ 1.8 keV 6030.6 -+ 1.8 keV 422.28 -+ 0.23 ms 7346.3 +- 10.2 422,69 +- 0.23 ms (0.29-+ 0.1)% 3096 -+ 6 s
a) Following the calculative procedures of ref. [2]. 123
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References [1 ] R.J. Blin-Stoyle, Fundamental interactions and the nucleus (North-Holland, Amsterdam) 1973. [2] D.H. Wilkinson, Nature 257 (1975) 189. [3] J.C. Hardy and I.S. Towner, Nucl. Phys. A254 (1975) 221. [4 ] S. Raman, T.A. Walkiewicz and H. Behrens, Atomic Data and Nuclear Data Tables 16 (1975) 451. [5 ] J.M. Freeman, G. Murray and W.E. Burcham, Phys. Lett. 17 (1965) 317. [6] A. Rytz, Atomic Data and Nuclear Data Tables 12 (1973) 487.
124
8 November 1976
[7] J.C. Hardy et al., Phys. Rev. Lett. 33 (1974) 320. [8] S.D. Hoath et al., Phys. Lett. 51B (1974) 345. [9] J.C. Hardy and I.S. Towner, Atomic masses and fundamental constants 5, eds. J.tt. Sanders and A.H. Wapstra (Plenum Press, London, 1976) 66. [10] P. Gl~ssel et al., Atomic masses and fundamental constants 5, eds. J.H. Sanders and A.tt. Wapstra (Plenum Press, London, 1976) 110. [11] D.H. Wilkinson and D.E. Alburger, Phys. Rev. C13 (1976) 2517. [12] D.C. Robinson, Nucl. Instr. 79 (1970) 65. [13] W. Jaus, Phys. Lett. 40B (1972) 616.
PttYSICS LETTERS
THE ft VALUE OF THE SUPERALLOWED
8 November 1976
F E R M I D E C A Y 46 V(fl+ )46Ti
G.T.A. SQUIER and W.E. BURCHAM Physics Department, University of Birmingham, UK S.D. HOATH* Nudear Physics Laboratory, Oxford, UK and J.M. FREEMAN, P.H. BARKER** and R.J. PETTYt Nuclear Physics Division, AERE, Harwell, Oxon, UK Received 15 September 1976 The 46Ti(p, n)46V reaction threshold has been measured as 8007.7 + 1.8 keV, equivalent to an energy release of 6030.6 + 1.8 keV in the beta decay 46V(fl+)46Ti. The 46V half-life has been measured as 422.28 + 0.23 ms. The corresponding ft value is reasonably consistent with those of other superallowed Fermi decays.
The conserved vector current hypothesis of basic weak interaction theory predicts that, for all superallowed Fermi decays between j~r = 0 +, T--- 1 analogue states, t h e f t values should be the same after correction for nuclear charge-dependent and electromagnetic radiative effects [1 ]. Within the uncertainties in the theoretical corrections, and the experimental errors in the measurements of the beta end-point energies and half-lives, on which the f t values depend, recently published surveys [ 2 - 4 ] of existing data conclude that the criterion of equality of the f t values appears to be met to within a few tenths percent. However, a close inspection of the present experimental situation in the case of the decay 46V(/3+)46Ti shows that there remain some questions concerning both its beta end-point energy and half-life; these will be discussed in this paper. Considering first the beta end-point energy E O, we note that the first result published for this with a quoted error < 0.05% was that obtained from a measurement at Harwell [5] of the threshold for the 46Ti(p, n)46V reaction. From this threshold the equivalent E 0 was deduced as 6032.1 -+ 2.2 keV. This becomes 6031.4 -+ 2.2 keV using the present value of the 212po c~-particle energy standard [6]. * Now at the Rutherford Laboratory, Chilton, Oxon, UK. ** On leave from the University of Auckland, New Zealand. Now at the Physics Department, University of Melbourne, Australia. 122
More recently Hardy et al. [7] have derived the result 6018.8 -+ 2.8 keV for the 46V end point energy E 0 from their measurement of the Q-value for the reaction 46Ti(r, t)46V relative to that for 27A1(~-, t)27Si. In discussing the large discrepancy with the earlier E 0 value, the authors pointed to the possibility of compoundnuclear resonances having affected the (p, n) threshold measurements on which the earlier result was based. Such effects can indeed occur as we have found, for example, in recent studies of the 54Fe(p, n)54Co threshold [8]. The Chalk River result for 46V has therefore been adopted in the latest compilations of superallowed Fermi decay data [ 2 4 ] , and when combined with the accepted value for the 46V half-life, it has led to anft value showing good consistency with the average for all such decays. However, Hardy has since reported [9] an error of several standard deviations in the Chalk River endpoint value for 46V, attributed to a contaminant peak in the (r, t) measurement. This has been confirmed in a measurement of the same (r, t) reaction Q-value by Gl~/ssel et al. [10]. Their preliminary result corresponds to an E 0 value of 6029 + 2 keV, consistent with the result from the earlier (p, n) threshold measurement. In the meantime we have been making a fresh study of the 46Ti(p, n)46V threshold, applying the tech-
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PHYSICS LETTERS
niques used in the case of 54Fe(p, n)54Co. The procedure followed was that of repeated irradiation of the target (400/ag/cm 2 of 81% 46Ti on a gold backing) by proton beam bursts of duration about a half-life (0.4 s for 46V), the relative reaction yield being followed as a function of proton energy by detecting positrons from 46V in a plastic scintillator, between the beam bursts. Positron pulses were accumulated in a 128channel multiscaler with 0.02 s channel stepping, for several hundred irradiation cycles at each proton energy, and the short-lived component was extracted from the observed decay curve. In this way the 46V yield could be followed down to a level of around 10% that of the background. The method provided a sensitive means of checking the shape of the yield curve in the threshold region, thus allowing the possible presence of resonance effects to be investigated. The yield curve was analysed according to the thick target 3/2 power law over a range of ~7 keV above the energy at which 46V activity was first detected; beyond this range the effect of a resonance was evident. The proton energy scale was calibrated as previously described [8] in terms of the 8784.37 -+ 0.07 keV a-particle group from a 212po source [6]. Seven independent threshold measurements were made, and the mean result, with allowance for discrete energy loss in the target and finite beam energy spread was 8007.7 k 1.8 keV, leading to a - Q value of 7835.2 -+ 1.8 keV, and an end-point energy for the 46V(/3+)46Ti decay of 6030.6 + 1.8 keV. This result is consistent with the earlier Harwell measurement and also with the result reported by the Munich Group [101. The value for the 46V end point from the present experiment, taken in conjunction with the recommended value of 424.4 + 1.4 ms [3, 4] for the halflife, corresponds to an ft value for this superallowed Fermi decay, which is more than two standard deviations higher than the weighted mean o f the other seven most precisely determined ft-values. Very recently Wilkinson and Alburger [11 ] have reported a new measurement of the 46V half-life, giving the result 424.01 + 0.47 ms. With this more precise value the discrepancy between the 46V d e c a y f t value and the mean fTvalue becomes more marked. Following the calculative procedure of ref. [2], these are 3109 + 6 s and 3088 -+ 2 s, respectively. The absolute ft values depend to a small extent on the values chosen
8 November 1976
for the charge-dependent corrections [2, 3, 4] but the discrepancy remains. However, we have also been carrying out an independent study of the 46V half-life, using the 46Ti(p, n)46V reaction on the enriched 46Ti target described above, at a proton energy of 8.15 MeV corresponding to a resonance not far above the reaction threshold. We have paid careful attention to possible sources of systematic error, particularly from contaminant activities and from pulse pile-up effects, using the method of decay curve analysis described by Robinson [12]. The only significant contaminant found was a 4 s activity with ~4 MeV end point, identified as 27Si from the (p, n) reaction on 27A1. This had a discernible effect on the deduced half-life for 46V in some of the runs, and these were analysed by a double-exponential fitting program. The effects of pulse pile-up were deter. mined experimentally, and small corrections were found necessary in some of the runs. Further details of the measurements and analysis will be reported elsewhere. The weighted mean of groups of runs made under a variety of conditions was 422.28 ms, with an internal statistical error of +0.23 ms, which was slightly greater than the external scatter error and is therefore adopted. This result is significantly lower than the other measurement of comparable precision, that of Wilkinson and Alburger [11] quoted above. The results of the present measurements are set out in table 1. The effective f t value for 46V decay is calculated following the procedure of Wilkinson [2], with ft =fR t(1 -- e) where fR is the integrated Fermi function corrected for outer electromagnetic radiative effects up to order Z2c~3 [13], t is the half-life corrected for electron capture, and e is the charge-dependent correction. The calculated ft value for 46V decay is reasonably consistent with those for the other superallowed Fermi decays. Table 1 The ft value for the decay 46V(~+)46Tifromthe present work
46Ti(p, n)46Vthreshold Q value
46V(fl+)46Tiend-point energy half-life
fR a) t e a)
ft
8007.7 -+ 1.8 keV -7835.2 -+ 1.8 keV 6030.6 -+ 1.8 keV 422.28 -+ 0.23 ms 7346.3 +- 10.2 422,69 +- 0.23 ms (0.29-+ 0.1)% 3096 -+ 6 s
a) Following the calculative procedures of ref. [2]. 123
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References [1 ] R.J. Blin-Stoyle, Fundamental interactions and the nucleus (North-Holland, Amsterdam) 1973. [2] D.H. Wilkinson, Nature 257 (1975) 189. [3] J.C. Hardy and I.S. Towner, Nucl. Phys. A254 (1975) 221. [4 ] S. Raman, T.A. Walkiewicz and H. Behrens, Atomic Data and Nuclear Data Tables 16 (1975) 451. [5 ] J.M. Freeman, G. Murray and W.E. Burcham, Phys. Lett. 17 (1965) 317. [6] A. Rytz, Atomic Data and Nuclear Data Tables 12 (1973) 487.
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[7] J.C. Hardy et al., Phys. Rev. Lett. 33 (1974) 320. [8] S.D. Hoath et al., Phys. Lett. 51B (1974) 345. [9] J.C. Hardy and I.S. Towner, Atomic masses and fundamental constants 5, eds. J.tt. Sanders and A.H. Wapstra (Plenum Press, London, 1976) 66. [10] P. Gl~ssel et al., Atomic masses and fundamental constants 5, eds. J.H. Sanders and A.tt. Wapstra (Plenum Press, London, 1976) 110. [11] D.H. Wilkinson and D.E. Alburger, Phys. Rev. C13 (1976) 2517. [12] D.C. Robinson, Nucl. Instr. 79 (1970) 65. [13] W. Jaus, Phys. Lett. 40B (1972) 616.