Identification of a 6.6s isomeric state in Ir175

An experiment has been performed to study excited states in the neutron-deﬁcient nucleus 175 Ir via the use of the JUROGAM II HPGe array and the RITU gas-ﬁlled separator at JYFL, Jyv¨askyl¨a. By using isomer tagging an isomeric state with a half-life of 6.58(15) µ s has been observed in 175 Ir for the ﬁrst time. It has been established that the isomer decays via a 45.2(E1) – 26.1(M1) keV cascade to new states below the previously reported ground state in 175 Ir with I π = (5/2 − ). We now reassign this (5/2 − ) state to the isomeric state discovered in this study.


I. INTRODUCTION
The neutron-deficient nuclei in the lead region provide a rich interplay of different nuclear structure phenomena, including low-energy shape coexistence [1,2]. Inbeam γ-ray spectroscopy is one of the most extensively used methods to study excited states, and usually works well for even-even nuclei. However the application of this technique to odd-A (and odd-odd) nuclei is often hampered by the presence of multiple competing bands and their complexity, in comparison to even-even nuclei. Furthermore, due to the existence of isomeric states with lifetimes longer than the typical 'prompt' time window usually applied in the analysis of γ-ray data, so-called 'floating bands' are often observed, whereby the interconnecting band transitions and/or transitions to the ground state are unobserved. As a result it is difficult to perform a quantitative analysis of the bandhead energy systematics in odd-A and odd-odd nuclei.
This work concentrates on 175 Ir which has been previously studied via prompt in-beam spectroscopy by means of fusion-evaporation reactions and the use of the HPGe γ-ray detector arrays POLYTESSA [3] and CAESAR [4]. *  The results of both investigations mostly agree with each other, and they identified several bands, some of which are floating, for example a strongly-coupled π9/2 − [514] band with the 9/2 − bandhead. While some disagreement remains about the exact configurations of these bands, a I π = (5/2 − ), π1/2 − [541] ground state was proposed by both studies which is also quoted by NNDC [5]. This assignment was based on the systematic properties of odd-proton nuclei in this mass region, on measured angular anisotropies for the strongest γ rays and on total-Routhian-surface calculations [3,4]. The in-beam investigations were however, insensitive to delayed transitions and the possibility that some of the observed bands are built on top of isomeric states cannot be excluded. Indeed, earlier β-, γ-and α-decay studies have suggested the existence of long-lived isomeric states in this nucleus. Two β-decay studies of 175 Ir deduced comparable halflives of of 13(2) s [6] and 11(3) s [7]. Somewhat shorter values were obtained via α decay measurements, T 1/2 = 4.5(10) s [8] and 7.2(13) s [7] 1 . The discrepancy between half-lives measured by different techniques was suggested to be due the existence of two isomeric states, a lowspin and high-spin state, which preferentially decay via α decay and β decay, respectively [6]. We note that the tabulated half life, T 1/2 ( 175 Ir) = 9(2) s [5] was evaluated from the weighted average of all measurements quoted above. To our knowledge, the most recent α-decay study of 175 Ir identified decays from two states with the following properties: the E α = 5395(5) keV, T 1/2 = 8.1 (5) s, originating from a 9/2 − state and the new decay at E α = 5745 keV, T 1/2 = 4.9(4) s from the (5/2-) state [9]. The study [9] suggested that these states should be associated with the same (5/2 − ) and 9/2 − states as proposed by in-beam investigations [3,4], but re-assigned the previously-known 5395 keV decay, proposed to decay from the (5/2 − ) state by the earlier work [7], to proceed from the 9/2 − state. However, the most recent EC/β +decay study of 175 Ir claims the observation of a 33(4) s β-decaying state, which could correspond to the 9/2 − bandhead [10]. The aforementioned inconsistencies between different studies show that the low-energy decay pattern of 175 Ir is far from being understood.
Furthermore, and relevant to the present discussion, a recent α-decay and hyperfine structure study of 179 Au, performed at ISOLDE, unambiguously established the ground state as I π ( 179 Au) = 1/2 + [11]. Based on the unhindered nature of the 5848 keV α decay 179 Au gs → 175 Ir [12], a spin and parity of I π = 1/2 + must be attributed to the state in 175 Ir fed by this decay. Therefore, a question arises on the relative position of this 1/2 + state in 175 Ir identified by α decay and the (5/2 − ), 9/2 − states proposed by earlier studies.
In order to investigate the low-energy structure of 175 Ir, we have performed an investigation of 175 Ir which allowed a search for low-energy isomeric transitions.

II. EXPERIMENT
An experiment was performed at the K-130 cyclotron at the Accelerator Laboratory of the University of Jyväskylä to study excited states in 175 Ir using the complete-fusion evaporation reaction 88 Sr + 92 Mo → 180 Hg * , in which 175 Ir was produced in the α, p evaporation channel. A 381 MeV beam of 88 Sr impinged on a 600 µg/cm 2 -thick self-supporting enriched 92 Mo target (of 98 % isotopic enrichment). Evaporation residues (ER) were separated from the primary beam and fission products using the gas filled separator RITU [13] and implanted in the Double Sided Silicon Detector (DSSD) of the GREAT spectrometer [14]. A segmented planar germanium detector (hereafter referred to as PGD) was used to detect x rays and low-energy γ rays at the focal plane of RITU. The prompt γ rays from the de-excitation of ER at the target position were detected with the JUROGAM II array (JGII) comprising of 23 EUROGAM clover detectors [15] and 15 EUROGAM phase one detectors [16]. The data were collected and timestamped using a Total Data Readout (TDR) data acquisition system [17] and sorted with the GRAIN software [18] to correlate events detected with the JGII and PGD.

III. DATA AND ANALYSIS
A. Identification of a 6.6 µs isomer in 175 Ir As the statistics collected in our experiment for 175 Ir was lower in comparison to the dedicated in-beam studies [3,4], we could not improve on the prompt in-beam decay scheme known from previous studies. However, the JGII data were useful for the assignment of the new isomeric state in 175 Ir, identified in our study, as shown below.
As a first step in our analysis, we concentrated on the identification of microsecond isomeric transitions at the focal plane of RITU, which were measured with the PGD within a specific time interval after the ER's implantation in the DSSD. The respective γ-ray spectra are dubbed as 'recoil-gated isomeric' spectra in the following text.
One of the most abundantly-produced reaction products in this study is 179 Au, produced in the (1p,0n) channel, for which an 89.5 keV isomeric state with a half life of 328(2) ns is known [19]. To remove the sub-µs isomeric transitions from the analysis and to get cleaner PGD γray spectra, we exclude events detected within 3 µs of a recoil implantation in the DSSD. Fig. 1a shows the recoil-gated isomeric γ-ray singles PGD spectrum (black line) with the requirement that events occur within the time interval of ∆T (PGD-DSSD= 3 -20 µs after the implantation of an ER in the DSSD. The spectrum drawn by a red line shows the background time-random γ-ray decays in the PGD which occur prior to an ER implantation in the DSSD. Fig. 1b provides the background subtracted spectrum, in which apart from two peaks at 45.2(2) and 147.4(4) keV, a somewhat broad structure with the strongest contribution at ≈66 keV is seen. The 147 keV isomeric transition is known in the isotope 177 Pt [20], produced in the (2p,0n) channel of the studied reaction. By applying an exponential fit of the time distribution between an ER implantation in the DSSD and the detection of a 147 keV γ ray in the planar detector, a half-life value of T 1/2 (147 keV) = 2.35(4) µs was deduced. It is consistent with the literature value of 2.2(3) µs [20], but more precise. The 66 keV peak corresponds to the Pt K α x rays due to the internal conversion of the 147 keV decay, the less intense K β x rays are masked by the non-fully subtracted background around these energies.
By using the same fitting procedure as applied for the 147-keV decay of 177 Pt, a half-life value of T 1/2 (45.2 keV) = 6.58 (15) µs was extracted for the 45.2 keV transition within the time interval of 3 -20 µs after the implantation of an ER in the the DSSD. The fit function consists of an exponential function and a constant background, with the result shown in the inset of Fig. 1b. The constant background in the fit function is used to account for γ rays not correlated to an ER and which have a flat time distribution.
To assign the transition to a specific nucleus, we used the Isomer Decay Tagging (IDT) [21] method in which isomeric decays detected in the PGD are correlated to prompt γ-ray transitions detected in JGII. Fig. 2a shows the prompt γ-ray singles spectrum detected in JGII, tagged on the 45.2 keV isomeric transition in the PGD. A time gate of ∆T (ER-45.2 keV)= 3 -20 µs was also applied for the PGD. The spectrum shows a large number of γ rays previously assigned to 175 Ir, above the (5/2 − ) state that was proposed as the ground state [3,4]. The above arguments unambiguously establish 175 Ir nuclide as the origin of the 45.2 keV isomeric decay. Fig. 3 shows a partial decay scheme, based on the drawings of [3,4], for states relevant to the present discussion. Despite the lower statistics in our data, relative to [3,4] our decay scheme above the (5/2 − ) state is consistent with the previous studies (cf. Fig. 4 in [4] and Fig. 1 in [3]). We clearly see the (1/2 − )[541] and (1/2 + )[660] bands 2 [4], which decay to the previously proposed (5/2 − ) ground state. Furthermore, the observation of the 197 and 231 keV transitions, which are the lowest-lying observed transitions in the bands and both decay to the (5/2 − ) state according to [3,4], suggest that the isomeric state which decays via the 45.2 keV transition must be situated below the (5/2 − ) state.
Based on the energy sum balance between different γ ray transitions, the previous studies [3,4] deduced the ne-cessity for the (9/2 − ) state at 49 keV above the (5/2 − ) state, but did not report the direct observation of such a transition. Based on our data, namely that the isomer sees the whole 1/2 − [541] band, including the lowest 230 keV transition, such a branch should indeed exist. However, due to its strong conversion (α tot (49, E2) = 117.7(17) [22]) and very low efficiency at this energy in JGII, we were unable to observe this transition, therefore it is shown in Fig 3 by the dashed arrow. On the other hand, the (9/2 − )[514] and (5/2 + )[402] bands [4] are unobserved in Fig. 2a. Transitions within these bands are observed, however in the ER-JGII γγ spectra without a PGD energy gate, notably the 145 keV transition. There is disagreement between the placement of this transition in the previous studies, Cederwall et al. [3] assigns this γ ray transition to the de-excitation of the (13/2 + ) state into the (5/2 + )[402] band, whereas Dracoulis et. al [4] places this transition within the band. Irrespective of this ambiguity we consider this transition as a signature of the (5/2 + )[402] band. In our work the 145 keV transition is seen with equal intensity to the 161 keV transition (17/2 + → 17/2 − in Fig. 3) in the ER-JGII γγ spectra, but is absent in the [ER-JGII γγ ]-PGD γ(45.2) spectra, which shows that the (5/2 + )[402] band does not feed the state decaying by the isomeric 45.2 keV decay.
Further analysis of the 45.2 keV decay was performed by searching for coincident transitions within the PGD, with the time conditions ∆T (ER − 45.2keV ) = 3-20 µs and ∆T (γ-γ) ≤ 50 ns. Fig. 4 shows the projection of this γ-γ matrix with a gate on the 45.2 keV decay, in which a transition at 26.1(4) keV is clearly seen. This coincidence establishes an isomeric cascade of 45.2 -26.1 keV transitions in 175 Ir. The relative order of these transitions will be discussed in the next section.

A. Multipolarities of New Transitions
Multipolarity determination for the 45-keV and 26-keV transitions was performed by measuring their total internal conversion coefficients, α tot . They are determined from the ratios of intensities of selected prompt transitions within the excited bands of 175 Ir measured in JGII with and without a gate on the isomeric transition in PGD. Moreover, to improve the cleanliness of the spectra, we used the recoil-and prompt γ-γ gating in JGII with a gate on 249-keV or 372 keV transitions, which also allow the selection of a specific part of the decay scheme. For example, for the case of the 372 keV transition, we compared the projections from the ER-γγ(372,JGII) and [ER-γ-γ(372,JGII)]-γ(45.2 keV, PGD) matrices, shown in Fig. 2b and in Fig. 2c  in the analysis applied below. Following this approach, and by using the 372 keV transition in JGII, an experimental value of α tot,exp (45.2) can be determined from: where N γγ(372) is the number of counts for a specific prompt γ ray in JGII with a gate on 372 keV without an isomeric gate in PGD (Fig. 2b) and N γ(372)γ(45.2) is the same number, but with a gate on the 45.2 keV γ-ray transition in the PGD (Fig. 2c). A value of (45.2) = 0.225 (20) is the efficiency of the planar detector at 45.2 keV, taken from simulations [23]. Table I shows Table I. All four values are in agreement with each other within the experimental uncertainties with the weighted average of α tot,exp (45.2 keV)=0.72 (7). This experimental value matches well to the theoretical value of α tot,th (45.2 keV)=0.676 (13) for an E1 multipolarity, calculated from [22]. For comparison, all other multipolarities would lead to much higher values, e.g. α tot,th (45.2 keV, M1) = 10.92 (21). Based on this value, an E1 multipolarity was unambiguously assigned to the 45.2 keV transition.
As mentioned in Sec. I, the unhindered α decay of the 1/2 + ground state in 179 Au established the existence of the state with I π =1/2 + in the daughter 175 Ir nucleus with the same structure as in the parent nuclide. Based on the multipolarities deduced for the 45.2 and 26.1 keV transitions, being E1 and M1, respectively, we note that these two γ ray transitions would fit the possible decay sequence (5/2 − ) →1/2 + via an intermediate 3/2 + state at 26.1 keV above the 1/2 + state. On these grounds, we propose that this isomeric cascade feeds to the same 1/2 + state as was identified in the α-decay study, as shown in A comparison of the decay of the 6.6 µs isomer observed in 175 Ir with the decay of a 326 ns isomer in 179 Au [19] showing the similar decay paths.
the decay scheme drawn in Fig. 3. Tentatively, the 1/2 + state could be the ground state in 175 Ir, but no further arguments could be provided to support this statement .
It is interesting to stress a striking similarity between the decay of the 89.5 keV, 328 ns isomeric state in 179 Au [19,25], and of the 71.3 keV, 6.6 µs isomeric state in 175 Ir which is shown in Fig. 5. In both cases, a cascade of E1-M 1 γ rays was observed, with the second decays in the cascade from the, presumably, (3/2 + ) excited states at 27.1 keV ( 179 Au) and 26.1 keV ( 175 Ir) being of the (3/2 + → 1/2 + ), M1 character. This fact might further strengthen the suggestion that both the 1/2 + ground state of 179 Au and the 1/2 + state in 175 Ir have the same structure, as was already proposed based on unhindered α decay between both states.

V. OUTLOOK
The rather long-half life of the 71.3 keV state is the most probable reason for the non-observation of its decay in the previous prompt in-beam studies [3,4], which presumably used a much narrower time window for γ-γ coincidences 3 . Furthermore, due to their low energies, both γ rays suffer from internal conversion (especially the 26.1 keV transition) and reduced γ-ray detection efficiency. The use of a highly-efficient segmented planar germanium detector in our study allowed us to overcome the latter issue, while also providing γ-γ coincidences.
The low-energy part of the excitation spectrum of 175 Ir is however, still far from being understood, as was already stressed by the studies [3,4] and further reiterated in Sec. I. In the present study, the excited states within three out of six bands, proposed by the previous studies could be observed built on top of the newly-reassigned (5/2 − ) state, which is now recognised as a 6.6 µs isomeric state at 71.3 keV above the presumed 1/2 + ground state. While the transitions within the floating band built on the high-I orbital (9/2 − in [4], 11/2 − in [3]) are also seen in prompt recoil-gated γ-γ data in JGII, they are not observed when gating on the 45.2 keV isomeric transition. This fact might indicate that the decay of this band either by-passes this isomer, or it ends up in a long-lived α and/or β-decaying state with a spin value very different from the tentatively-proposed I π =1/2 + ground state in 175 Ir. The latter was also hinted at by β-decay studies of 175 Ir [7] where a strong 105.6 keV transition is observed in 175 Os [6,7]. This γ ray was also seen in an in-beam study of 175 Os and assigned to a (7/2 + ) → (5/2 − ) transition [27] which suggests the existence of a high spin β-decaying state in 175 Ir, which could potentially correspond to the 9/2 − [514] bandhead, see also the results of the most recent EC/β + -decay study [10].
We also note that our study bears further insight on the most recent α-decay study of 175 Ir [9] which reassigned the earlier known 5395 keV α decay to the 9/2 − state (instead of 5/2 − ), and proposed a new 5745 keV decay as originating from the (5/2 − ) state. In view of the results of the present work, one would need to assume that the 5745 keV decay actually originates from the 1/2 + state, rather than from (5/2 − ) state.
Questions also remain about the (5/2 + )[402] band [4] (band 6 in [3]). The previous studies disagree about the level ordering within this band and the current study is unable to resolve these discrepancies.
A further investigation of this interesting nucleus is in order, including dedicated α-and β-decay studies.