Measurement of the Excited Energies Identified in 42Ca using the ROSPHERE Gamma-ray Arrays

Excited energy states totalling 11 in number identified to be associated with the 42Ca were detected via the ROSPHERE gamma-ray array detectors of IFIN-HH Bucharest from a 28Si(18O, 2p2n), where a multi-particle of 2 protons and 2 neutrons were evaporated in a fusion reaction. The excited energies were identified using γ-ray coincidences. All detected gamma energies (γ) recorded were compared with various literatures from NNDC and this shows an excellent agreement with each and the results are presented together with their calculated relative intensities.


Introduction
Calcium-42 is one of the isotopic-nucleus of calcium with 22 neutrons and 20 protons. The two excess neutrons present in the nucleons distinguish it from the first stable nucleus of 40 Ca with either protons or neutrons being magic. The heaviest of the isotopes is 48 Ca with 8-neutrons higher than 40 Ca [1]. The measured gamma energies of the nucleus have been reported in several articles with some of them showing the relative intensities of the measured gamma transitions. In this, the relative intensities of these gamma energies have been measured after they were identified to be associated with 42 Ca through the instrumentalities of Coincidence analysis [2].

Experimental Details
In the Coulomb unsafe reaction of 192 Os target with the 18 O beam at 80 MeV using the 9 MV tandem accelerator of the Horia Hulubei National Institute of Physics and Nuclear Engineering (known as IFIN-HH) in Bucharest [3], a fusion multiparticle of 2n2p was evaporated from the fusion of 28 Si target with 18 O -beam. 28 Si was found to be one of the impurities in the 192 Os target foil material bought from the Trace company in USA. This contaminated percentage of 42 Ca and other chemical impurities from the target were observed to have reacted during the experiment when the 18 O beam delivered the laboratory energy of 80 MeV on the 192 Os target in the chamber of the accelerator. Details of the purity composition of the 192 Os are as shown in Table 1.
This nuclear fusion evaporation reaction produced 42 Ca as the product of the reaction after evaporating 2n2p particles from the compound with the beam current of 20 pnA with a hardware trigger condition of either LaBr 3 (Ce) -LaBr 3 (Ce) or HPGe -HPGe [4,5]. The predicted fusion-evaporation cross-section for the observed channels (that is, from 60 MeV to 100 MeV) have 42 Ca nucleus dominating the entire laboratory energy region as chosen in this work. The relative cross-section measured for the 42 Ca nucleus is ≤ 100 mb [4,5].
The experiment for the 18 O beam on 192 Os target was conducted for a period of 9 consecutive days at IFIN-HH Bucharest within which an approximate count of 1,200,000.00 was recorded using the ROmanian SPectroscopy in HEavy REactions, RO-SPHERE gamma-ray array of 14 High Purity Germanium, HPGe and 11 Lathanum-Bromide doped Cerium , LaBr 3 (Ce) detectors [4,5,6].

Data Analysis
Coincidence analysis was performed on the detected gamma energies from the ROPSHERE gamma-ray array detectors of the IFIN-HH Bucharest and these were identified to be associated with 42 Ca. Doing this, a gating condition was used where a particular gamma transition was selected as a reference energy and all other energies that were seen to be in coincidence, that is, appearing with the selected gate at the same time were recorded. This was to further confirm that the measured gamma transitions were indeed associated with 42 Ca [1, 7,8]. Table 2 shows all the measured gamma transitions in the current work. The partial energy level scheme of 42 Ca was obtained as shown in Figure 1 using RADware.

The Measured Energy Levels Associated with 42 Ca in the Current Work
The 1525-keV Level. The 1525 keV level is fed with a 1227 keV transition from the yrast I π = 4 + state to the I π = 2 + . This energy level is assigned a spin of 2 with a positive parity. It decays to the ground state 0+ with the gamma energy of 1525 keV. The relative intensities of the gamma transitions have been determined in this current work as shown in Table 2.
The 2752-keV Level. This energy level is populated by 437 keV gamma transition which is a decay from the 3190 keV level to a spin and positive parity of 4 + . The relative intensity is calculated to be 50 (even though the directional correlations from oriented states DCO's calculation is not shown here). The multipolarity is dominantly an E2, but can possibly be an M3 transition using the expression in equation (1): where the largest possible value of ∆L is I i + I f and the lowest possible value I i − I f . The parity change in the transition is given by the selection rules [9].
The 3190-keV Level. As shown in Figure 2 of the current work, more than two gamma transitions populated this energy level but with different experimental branching ratios, B(M1)/B(M2) (not determined here). Apart from 910 keV transition energy populating this state, both 2956 keV and 2302 keV gamma transitions are not observed in this current work. This also forms the basis for which the gamma transitions are shown as broken lines and under brackets. The observed multipolarity is M1 which is the likely dominant one. This is so because there is a strong dependence of the transition rate on multipolarity where the lowest multipolarities are most likely to occur [9]. The relative intensity of the 910 keV gamma transition is calculated as 18 and this is shown in Table 2.

Discussion and Conclusion
The coincidence spectra on selected gamma transition energies in the left panel of Figure 1 is a double fold: that is, γγ matrix sorting condition in the master trigger of ROSPHERE. In this sorting, the gamma energy generally has more counts as recorded for same gates in triple fold gating condition.