At the IGISOL facility coupled to the K130 cyclotron in Jyväskylä, highly neutron-rich isotopes of Y, Nb, and Tc -lying at and beyond the boundary of previously known nuclei- have been produced by fast fission of natural uranium with a 50 MeV H₂⁺-beam [1]. A comprehensive list of the T_1/2 and P_n-values is given in [2]. For the isotopes ¹⁰³Y and ^108-110Nb, beta-decay is reported for the first time. Altogether 13 new P_n-values have been measured, for the elements Nb and Tc for the first time. In addition, the P_n-values for ^99-102Y could be remeasured and show rather good agreement with literature values [3,4]. But at IGISOL also refractory elements are obtained as primary beams, whereas up till now Y activities had to be fitted as third decay component of primarily separated Rb/Sr with much higher delayed-neutron yields, leading to uncertain results.

Gross beta-decay properties, such as T_1/2 and P_n-values, are the easiest measurable quantities for isotopes very far from stability produced with low production yields. Although being integral parameters, they do contain nuclear structure information [5]. In comparing experimental results with predictions from nuclear models, such as the the QRPA model for Gamow-Teller beta-decay [6], one obtains first indications on possible nuclear-structure features associated with high neutron excess.

Empirical formulae which correlate the P_n-values with the energy window available for neutron decay can be used as a check against gross beta-decay properties. In Fig. 1, our experimental P_n-values (full symbols) are displayed together with predictions from the QRPA (open symbol) as a function of (Q_beta - S_n)/(Q_beta - C) postulated by Kratz and Herrmann [7]. For the QRPA calculations, masses and ground-state deformations were taken either from the FRDM model [8] or from the ETFSI approach [9]. In general, there is good agreement between experimental and predicted values. They follow the straight line in Fig. 1 which results from a fit to all known precursors in the fission product region known up to 1986 [10]. The different slope of the dashed line representing a local fit to the 17 displayed values is mainly caused by the unexpected high P_n-values of the Nb isotopes. This is better reproduced in the ETFSI model than in the FRDM. The ETFSI approach predicts lower S_n-values leading to higher P_n. Such a lowering of S_n-values for very neutron-rich nuclei with 110 < A < 120 had, for example, been "requested" from astrophysical considerations [11].

References:

1. T. Mehren et al., Institut für Kernchemie, Universität Mainz, Jahresbericht (1994) 31.
2. T. Mehren et al., submitted to Phys. Rev. Lett.
3. P.L. Reeder et al., Proc. Specialists' Meeting on Delayed Neutron Properties, Sep. 1986, Univ. of Birmingham, England; ISBN 07044 0926 7, p. 37.
4. B. Pfeiffer et al., ibid. p. 75.
5. K.-L. Kratz et al., Nucl. Phys. A417 (1984) 447.
6. P. Möller, J. Randrup, Nucl. Phys. A514 (1990) 1.
7. K.-L. Kratz, G. Herrmann, Z. Phys. 263 (1973) 435.
8. P. Möller et al., At. Data Nucl. Data Tables 59} (1995) 185.
9. Y. Aboussir et al., ibid. 61 (1995) 127.
10. T.R. England et al., ibid. Ref. [3], p. 117.
11. K.-L. Kratz et al., Ap. J. 403 (1993) 216.
12. G. Audi and A.H. Wapstra, Nucl. Phys. A595 (1995) 409.

Fig. 1: Pn-values as a function of the energy window for delayed-neutron emission according to [7]. For Qbeta and Sn, experimental values from [12] were used where available, otherwise ETFSI predictions [9] were applied.

Remark:

Ref. [2] T. Mehren, B. Pfeiffer, S. Schoedder, K.-L. Kratz, M. Huhta, P. Dendooven, A. Honkanen, G. Lhersonneau, M. Oinonen, J.-M. Parmonen, H. Penttilä, A. Popov, V. Rubchenya, J. Äystö
Beta-Decay Half-Lives and Neutron-Emission Probabilities of Very Neutron-Rich Y to Tc Isotopes
Phys. Rev. Lett. 77, 458(1996)