THE BETA ISKAY OF *MM~: G~mow-Te~I~r ~~e~c~ing in fpshell nuclei

A TIC-W nuclide, 4”Mn, has been identified and its decay has been studied, using on-line mass separation of “C<‘@Ca, p3n) reaction producrs. The half-life of this nuclide was determined to be I50 (10) ms, and spectra of @-delayed protons and y-rays have been obtained. A branching ratio of 2.7 (12)x Io’-s was deduced for the emission of &delayed protons, while an upper limit of 6 x iO_” was determined for ~-deIayed a: particle emission. A partial level scheme is proposed for 4sCr on the basis of y-singles and ~~-~inc~dence data. The deduced Gamow-TeIIer strength function is oompared with predictions from shell model calculations suggesting a quenching factor of053 (17) for the lowest lying states. This result is discussed in comparison with other que~cbing factors from P-decay studies, in particular those obtained for mirror P-decays in the fp-shell, Radioactivity ‘%ln (p’p) [from ‘ZC(40Ca, p3n)]; measured ~-delayed Er, I,, Ey, &-coin, yy-coin, T,,s; deduced log.& %r deduced levels, f,, ~-branching. Plastic scinrillator, Ge $Lif, surface barrier Si detectors. On-line mass separation. NUCLEAR STRUCTURE 48V, *sCn; calculated levels. 44V 48Ca; calculated Gamow-Teller transition strength distrjbution. Sh& model. h4uch progress has been made in exploring the properties of proton-rich (2 > N) nuclei up to the titanium (Z = 22) isotopes, so that the ex~e~rne~ta~ frontier in this mass region lies among nudides with isospin projection T, =$(N -Z) = -2 and T, ==-3. By contrast, in the region above titanium relatively little is known about the ~rope~i~s of nuclei which are more proton-rich than Tz = -4. The only removed ’ Present address: Sapan Atomic Energy Research Institute, repayment of Radioisotopes, Tokai, Naka, Ibaraki 319-11, Japan. ’ Humboldt awardee 1985, permanent address: University of California, Department of Chemistry and Lawrence Berkeley Laboratory* Berkeley, California 94720, USA. ’ Present address: Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories, Nuclear Physics Branch, Chalk River, Ontario KOJ HO, Canada. a Present address: Warsaw University, Institute of experimental Physics PL-00-681 Warsaw, Potand. ’ Also GANIL, B.P. 5027, F-i4021 Caen Cedex, France. 6 On leave of absence from Michigan State University, Repa~rn~nt of Physics and A~t~~~orny, East Lansing, Michigan 48824, USA. 0375”9474/g7/$03.5~ @ EIsevier Science ~bl~shers B.V ~No~h-~olIalld Physics ~blisb~~g Division) 94 T. Sekine et al. / p-decay of JsMn T, = -1 nuclei with Z 2 22, whose decay has been (in part) characterized, are ? [ref. ‘)I and 46Cr [ref. ‘)I. Beyond these, only the Tz = -i series of /?-delayed proton precursors with A = 4n + 1 have been identified between 45Cr and 61Ge [ref. ‘)I due to their large P-delayed proton branches. One of the motivations for studying these nuclei is to reach and possibly pass the proton drip line, which is mapped rather completely “) up to the sd-shell ( 160 to 40Ca) nuclei with a few singular cases of proton radioactivity for heavier nuclei between 53C~m and 151Lu. In this mapping one would be able to make significant gains in the understanding of the structure of these nuclei by studying cases of ground-state proton radioactivity and of P-delayed emission of protons, two protons or n-particles. There is, however, an additional motivation for performing detailed measurements of the (P-delayed proton and y-ray) decay of such nuclides. The large QP values of nuclei with Z > N lead to strong population of the isobaric analogue state (IAS) in the daughter nucleus by superallowed Fermi P-decay (AS = AL = 0), and furthermore allowed Gamow-Teller (GT) P-decay (AS = 1, AL = 0) can be studied under less energy-window restrictions. To reduce the latter constraint is of obvious importance, together with a “complete” measurement of the p-strength, in probing the action of the UT (or GT) operator in nuclear P-decay. As an example of this, already more than a decade ago, Wilkinson ‘) noted the reduction (quenching) of the low-energy part of GT’ strength observed in p’ decay as compared to shell-model calculations, which can be interpreted as renormalization of the weak axial-vector coupling constant in the nuclear many-body medium (ga) relative to the value for the free-neutron decay (gA). According to a recent review 6), the global GT quenching for the middle of the sd-shell amounts to 0.58 (5), based on a comparison of measured P-strength data with results from large-basis shell-model calculations. In recent years, missing GTstrength was also found in studies of charge-exchange reactions such as (p, n), the observed fraction of the GT sum rule strength being 50-65% for Aa90, while there is considerable scattering of these fractional values for lighter nuclei [see ref. ‘) for a review]. It is outside the scope of the present paper to survey the extensive experimental and theoretical literature on GT quenching, or to quantitatively evaluate the possible nuclear (e.g., nucleon-hole excitations) and subnuclear (e.g., A-isobar excitations) contributions to the strength reduction 6-9). As far as the relevant P-decay data are concerned, improved measurements and calculations are clearly needed, e.g., for fp-shell (40Ca to 56Ni) nuclei or, more generally speaking, both inside a given shell-model region and for larger ranges of A. We present in this paper the first study of P-delayed proton and y-ray emission from the decay of the new nuclide 48Mn (T,,? = 150 ms), which was produced through the 12C(40Ca, p3n) reaction at the GSI on-line mass separator. 16 y-rays were assigned to the decay of 48Mn and incorporated into a partial level scheme of 48Cr up to its isobaric analog state at 5792.4 (6) keV. The GT strength distribution is deduced for T. Sekine et al. / p-decay of 4”Mn 95 this energy interval from y-ray data, and for excitation energies of 9 to 11.5 MeV from measuring P-delayed proton emission. The experimental results are compared with predictions from shell-model calculations both for 48Mn and for other nuclei. 2. Experimental techniques The isotope 48Mn was first identified lo) during ion source tests at the GSI on-line mass separator “). These measurements had originally been planned for testing some production aspects of deep-inelastic reactions. A 11.4 MeV/u 40Ca beam from the UNILAC at GSI Darmstadt was directed onto a 25 mg/cm2 n”fW foil acting as target and ion source window. In this experiment as well as in the one with a thinner tungsten window described below, reaction products are caught in a graphite catcher inside the source and are, after release from the catcher and extraction from the source, mass separated in a magnetic sector field as 55 keV beams of singly-charged ions. Finally, decay spectroscopy is performed on mass-separated samples, using various collector and detector arrangements. During this test lo), a series of isotopes close to the P-stability line was identified for elements between sulfur (2 = 16) and cobalt (2 = 27), which are presumably due to deep-inelastic 40Ca + W reactions. In addition some neutron-deficient isotopes were detected including 48Mn. This isotope is evidently produced by fusion-evaporation reactions between the graphite catcher and the 40Ca beam, decelerated to approximately 7.3 MeV/u after passage through the tungsten foil. In order to improve the P-singles, y-singles and py-coincidence data obtained during this test, measurements were extended to yield y-singles and yy-coincidence information up to high y-ray energies; in addition, a search for P-delayed emission of protons and cY-particles was performed. For these measurements we used a 11.7 MeV/u 40Ca beam and a FEBIAD-F ion source 12) with a 12.4 mg/cm2 thick tungsten window and a 27 mg/cm2 thick graphite target-catcher. The beam energy on target amounted to 9.8 MeV/u for the 48Mn measurements, while the production rates of neighboring proton-rich isotopes were partly determined for a higher beam energy. In order to investigate pand y-rays, the mass 48 beam was directed onto an aluminized mylar tape. After collecting for a preselected time period, the activity was transported within 230 ms over a distance of 30 cm into the counting position, equipped with a thin 4~ plastic scintillator for P-detection and a Ge(Li) detector for y-detection. In order to also detect y-rays of very short-lived isotopes, where decay losses during the transport of the tape may become appreciable, another Ge(Li) detector was mounted at the collection position. In addition to this Py detection system, another tape collector was used for ~yy measurements. In this case, the collector position was equipped with a thin 2~ plastic scintillator detector and two large-volume hyperpure germanium detectors 96 T. Sekine et al. / p-decay of “Mn with photopeak efficiencies of 29% and 30.5% for 1.3 MeV y-rays relative to a 7.5 cm 0 x7.5 cm NaI(T1) detector. The 29% detector was closely facing the tape and the plastic detector, while the second one was mounted in 90” geometry at a somewhat larger distance; the absolute photopeak efficiencies were approximately 2.4% and 0.8% of 4rr at 1.3 MeV y-ray energy. The tape was moved regularly out of the collection position for reducing long-lived (mass 48) activity. For charged-particle measurements, the mass-48 beam was implanted into a 43 p,g/cm’ carbon foil in front of a telescope. The latter consisted of two surface barrier detectors of 17 pm thickness, 50 mm2 area and 718 pm thickness, 450 mm2 area. In order to determine the branching ratio of P-delayed particle emission relative to the intensities of P-delayed y-rays, the latter were measured simultaneously in a Ge(Li) detector mounted close to the telescope. In order to determine half-lives from grow-in and decay of short-lived activities, the mass-48 beam was switched off periodically by means of an electrostatic device both for the telescope-Ge(Li) array and for the collection positions of the tape stations. Using the GSI data acquisition system GOLDA 2 [ref. 13)], coincidence data were recorded, together with a time mark, event by event by a PDP11/45 computer; singles spectra were accumulated simultaneously in multispectrum mode. 3. Experimental results In this chapter we present the measurements of positrons, p-delayed y-rays and P-delayed protons, leading to a partial level scheme for 4*Cr and to the determination of several logft values for 48Mn -+ 48Cr p-transitions. Observed source strengths and estimated overall efficiencies for 48Mn and some neighboring nuclei are compiled in table 1. The efficiency values represent rough approximations, since the production cross sections calculated on the basis of the HIVAP code 14) have not been verified experimentally. Fig. 1 shows as examples of mass-48 measurements a P-coincident y-ray spectrum up to 4.1 MeV. The singles y-ray spectra have of course better statistics, but are more complex due to detection of room background. After accounting for single and double escape peaks as well as for peaks due to summing with 511 keV annihilation radiation, and leaving the y-lines at 1660 and 2167 keV unassigned, 16 -y-transitions were ascribed to the decay of 48Mn. Energies and intensities of these transitions are listed in table 2; the intensities were corrected for yy-coincidence summing in an approximate way, using the level scheme as discussed below. Examples for y-ray decay measurements and for yy-coincidence data are shown in figs. 2 and 3; the corresponding results are compiled in table 2. Energy and intensity calibration of the -y-ray detectors were obtained up to 3.3 MeV by using 56Co and 15’Eu sources. In particular, an “internal” calibration was performed by recording a mass-48 y-ray spectrum in the presence of a 56Co T. Sekine et al. / p-decay of @Mn