Beta decay of exotic TZ =-1 ,2 nuclei : the interesting case of 56 Zn

The β decay properties of the Tz = -2, Zn isotope and other proton-rich nuclei in the fp-shell have been investigated in an experiment performed at GANIL. The ions were produced in fragmentation reactions and implanted in a double-sided silicon strip detector surrounded by Ge EXOGAM clovers. Preliminary results for Zn are presented. The Zn decay proceeds mainly by β delayed proton emission, but β delayed gamma rays were also detected. Moreover, the exotic β delayed gamma-proton decay was observed for the first time. The Zn half-life and the energy levels populated in the Cu daughter have been determined. Knowledge of the gamma de-excitation of the mirror states in Co and the comparison with the results of the mirror charge exchange process, the Fe(He,t) reaction (where Fe has Tz = +2), were important in the interpretation of the Zn decay data. The absolute Fermi and Gamow-Teller strengths have been deduced.


Introduction
Beta (β) decay is a powerful tool to investigate the structure of nuclei far from the line of β stability, giving direct access to the absolute value of the Fermi (F) and Gamow-Teller (GT) transition strengths, B(F) and B(GT), respectively.However the finite Q β value only allows access to states at low excitation energy.On the other hand, Charge Exchange (CE) studies, at intermediate beam energies and zero momentum transfer, allow the determination of relative B(GT) values up to high a Corresponding author: Sonja.Orrigo@ific.uv.es excitation energy [1,2].Thus β decay and CE reactions complement each other.Under the assumption of isospin symmetry, the β decay of proton-rich nuclei and CE reactions carried out on the mirror, stable nuclei can be combined to determine the absolute B(GT) strength up to high excitation energies.Absolute B(GT) values can be obtained even for GT transitions starting from extremely unstable nuclei if the β decay half-life T 1/2 and the Q β value are precisely known.This "merged analysis" [1,3] method has been successfully employed to study T z = -1, proton-rich nuclei in the fp-shell [1,3,4].

The experiment
T z = -1 and -2 nuclei in the fp-shell were produced in an experiment performed at the LISE3 facility of GANIL [5] in 2010.The experiment was focused on the study of 56 Zn.A 58 Ni 26+ primary beam with average intensity of 3.7 eμA was accelerated to 74.5 MeV/u and fragmented on a 200 μm thick, natural Ni target.The fragments were selected by the LISE3 separator, and implanted into a Double-Sided Silicon Strip Detector (DSSSD) surrounded by four Ge EXOGAM clovers for gamma (γ) detection.The DSSSD was 300 μm thick and had 16 X and 16 Y strips with a pitch of 3 mm, defining 256 pixels.The DSSSD was used to detect both the implanted ions and the subsequent β decays, using two parallel electronic chains with different gains.The implanted nuclei were identified on an eventby-event basis using the energy loss signal, measured in a silicon ΔE detector located before the DSSSD, and the Time-of-Flight, measured between the cyclotron radio-frequency and the ΔE signal.Any implantation event triggering the ΔE detector was acquired.Decay events were characterized by a signal above threshold (typically 50-90 keV) in the DSSSD with no coincident signal in ΔE.

Beta decay of the exotic 56 Zn isotope
The 56 Zn half-life was determined from the time correlation between each decay event happening in a given pixel of the DSSSD and all the 56 Zn implants occurring before and after it in the same pixel.The time difference between these two types of event defines the correlation time.From the correlations of 56 Zn implants with the protons (above 800 keV, see below), the half-life was obtained by fitting the correlation time spectrum with a function including the β decay of 56 Zn and a constant background from the random correlations.A half-life T 1/2 = 32.9 ± 0.8 ms was obtained for 56 Zn.
The decays of the T z = -2 nuclei are more complex than the T z = -1 cases (where only β delayed γ emission is observed) because both β delayed γ rays and β delayed proton (p) emission can occur because of the extremely low proton separation energy S p .In the T z = -2 → -1, β decay the 56 Zn nucleus decays to 56 Cu (Q β # = 12870 ± 300 keV [6], where # means from systematic).In the 56 Cu nucleus, S p # = 560 ± 140 keV [6] only.Thus it is expected that the energy levels populated in 56 Cu above S p will decay via p-emission to the 55 Ni ground state (gs).
Fig. 1a shows the charged-particle spectrum for the decay events correlated with 56 Zn implants, after subtraction of the random background.Most of the strength in the spectrum can be attributed to β delayed proton decays to the 55 Ni gs : four intense and two weaker proton peaks are identified above 800 keV (labeled according to the corresponding excitation energies in 56 Cu), while the bump below 800 keV could be attributed to β decays which are not in coincidence with protons.The thinner DSSSD detector used in the present experiment, as compared to a previous experiment by Dossat et al. [7], produced a better energy resolution for the protons, 70 keV FWHM.The 0 + Isobaric Analogue State (IAS) populated in the Fermi transition is clearly identified as the strong peak at 3508 keV (as in [7]).Most of the other levels correspond to the excitation of 1 + states in 56 Cu via GT transitions.Fig. 1b shows the spectrum obtained from the mirror T z = +2 → +1, 56 Fe( 3 He,t) 56 Co CE reaction measured at RCNP Osaka [8].A good correspondence is observed between the states in the two mirror nuclei, 56 Cu and 56 Co, with the energies differing by less than 100 keV.Here ref. [6] was used for the estimation of S p and Q β .Using ref. [9] the same comparison gives energy differences of ~ 400 keV.
It is worth noting that the proton decay of the T = 2, 56 Cu IAS state at 3508 keV (Fig. 1a) to the T = 1/2, 55 Ni gs is isospin forbidden, which makes the competing γ de-excitation possible.Indeed the γ  MeV), we have observed an exotic decay, namely a β delayed gamma-proton decay, for the first time.
Imposing coincidence conditions on the various proton peaks in Fig. 1a, two additional γ rays are observed (Fig. 2b, 2c).The first one is at 861 keV and corresponds to the de-excitation from the 3508 keV IAS to the 2661 keV state.The second lies at 309 keV and is related to the electromagnetic transition connecting the 1691 and 1391 keV states.Thus, the level at 1391 keV could correspond to the 0 + anti-analogue state [10] at 1451 keV in the mirror 56 Co and could be indirectly populated by the γ decay from the 1691 keV state, as occurs in the mirror 56 Co nucleus.
The 56 Zn decay scheme summarizing all of the observations from the present experiment is shown in Fig. 3.The 56 Cu level energies have an accuracy of ~ 10 keV from the determination of the proton peak centroids; the larger error indicated in Fig. 3 is due to the uncertainty in S p [6].Solid lines indicate experimental observations, while dashed lines indicate transitions observed in the mirror 56 Co nucleus.Three cases of β delayed γ-proton emission are seen experimentally, involving the levels at 2661, 1691 and 1391 keV and the γ rays at 861, 1835 and 309 keV.
The β feeding to each level populated in 56 Cu is estimated from the area of the proton peaks, corrected for the amount of indirect feeding produced by the γ de-excitation.The latter comes from the intensity of the observed γ lines and the de-excitation pattern in the mirror 56 Co nucleus [10].Assuming 100% DSSSD efficiency for both implants and protons [7], a total proton branching ratio of 88.5 ± 0.9 % is deduced by comparing the total number of 56 Zn implants with the number of protons observed above 800 keV (Fig. 1a).The missing 11.5 ± 0.9 % is attributed to the γ decay of the 1691 keV level, where the estimated partial proton half-life is t 1/2 ~ 10 -14 s, thus the γ de-excitation can compete with the p-emission (being γ the 56% and 66% of total decays from IAS and 1691 keV state).
The β feedings, T 1/2 , Q β [6] and total proton branching ratio were used to determine the B(F) and B(GT) values.The 56 Cu IAS at 3508 keV has a preliminary B(F) = 2.7 ± 0.5 units instead of the expected value of |N -Z| = 4.Besides the IAS, three other states at ~ 3.5 MeV are observed in the mirror 56 Co nucleus (Fig. 1b); one of them is also fed by the Fermi transition (like the IAS), the others by GT transitions.Fragmentation of the IAS has been observed in this mass region and in the case of

DOI: 10
.1051/ C Owned by the authors, published by EDP Sciences,

Figure 1 .
Figure 1.a) Charged-particle spectrum measured in the DSSSD for decay events correlated with 56 Zn implants.The peaks are labeled according to the corresponding excitation energies in 56 Cu (in MeV).b) The 56 Fe( 3 He,t) 56 Co reaction spectrum [8].The peaks are labeled by the excitation energies in 56 Co.

Figure 2 .
Figure 2. a) Gamma-ray spectrum measured in coincidence with the charged-particle-decays correlated with 56 Zn implants.b) γ rays coincident with the p-peak at 2661 keV.c) γ rays coincident with the p-peak at 1391 keV.spectrum measured in coincidence with the decays correlated with 56 Zn (Fig.2a) exhibits a line at 1834.5 ± 1.0 keV.This energy agrees with the difference between the 3508 and 1691 keV states, namely 1817 ± 15 keV, therefore this γ line is attributed to the electromagnetic transition connecting these levels.Further confirmation arises from the fact that the 1835 keV line is in coincidence with the p-decay from the 1691 keV level.Moreover, the half-life of the 1835 keV peak is T 1/2 = 27 ± 8 ms, in good agreement with the 56 Zn value.Since the 1691 keV level is also p-unbound (estimated Γ ~ 10 -8

Figure 3 .
Figure 3. Decay scheme of 56 Zn as deduced from the present experiment.Observed proton or gamma decays are indicated by solid lines.Transitions observed in the mirror 56 Co nucleus are shown by dashed lines.The level energies are deduced from our data, where the intrinsic uncertainty is estimated to be ~ 10 keV.The larger error is due to the uncertainty in the proton separation energy.