Study of the 32S(3He,d)33Cl one-proton transfer reaction with a new generation hodoscope

The 32S(3He,d)33Cl one-proton transfer reaction is a powerful tool to investigate the spectroscopy of low-lying states in the proton-rich 33Cl nucleus. However, the extraction of firm differential cross-section data at various angles, against which benchmarking theoretical models to correctly constrain the spectroscopy of 33Cl, is made challenging by the presence of competitive reaction products contaminating the detected energy spectra. We have recently measured the 32S(3He,d)33Cl reaction at 9.8 MeV incident energy by using a new generation hodoscope of silicon detectors, capable to detect and identify emitted deuterons down to energies of the order of 2 MeV. The high angular segmentation of our hodoscope allowed to unambiguously disentangle the contribution of one-proton transfer reactions in the ground state of 33Cl and in its 0.810 MeV, 2.352 MeV, 2.685 MeV, 2.846 MeV excited states from contaminant deuteron-emitting reactions. These data will be crucial to help to constrain Jπ and spectroscopic factor C2Sp values of low-lying 33Cl states, still ambiguous in the literature. The present status of the analysis is discussed in the paper.


Introduction
Nuclear reactions at low and intermediate energies allow to probe many properties of nuclear systems and their governing forces such as the structure of nuclei [1,2,3,4] and clustering phenomena [5,6,7], their influence in Astrophysics [8], the dynamics of nucleus-nucleus collisions [9,10,11], and their interplay [12]. While the latter requires often the use of 4π detectors [13,14,15] because of the complexity of the final state of the collisions, nuclear spectroscopy XLII Symposium on Nuclear Physics 2019 IOP Conf. Series: Journal of Physics: Conf. Series 1308 (2019) 012007 IOP Publishing doi: 10.1088/1742-6596/1308/1/012007 2 studies are typically more selective, enabling the use of more compact devices [16]. In this framework, transfer reactions are a class of particularly selective processes that allow to explore specific degrees of freedom in nuclear systems such as single-particle configurations. In the literature, several transfer reaction studies have been performed by using spectrometers like MAGNEX at INFN-LNS [17], VAMOS at GANIL [18] and K600 at iThemba LABS [19]. Recently, new high-resolution and modular detectors for nuclear physics have been developed (see, for example FARCOS at INFN-LNS [20] or HiRA10 at MSU-NSCL [21]) to be used for nuclear reactions or as ancillary systems for large acceptance devices [22,23] or magnetic spectrometers [18,24]. Such detectors have the advantage of high angular segmentation, high energy resolution and a high degree of modularity. In this paper, we exploit the new modular device OSCAR [25] to investigate the one-proton transfer reaction 32 S( 3 He,d) 33 Cl at 9.8 MeV incident energy. This reaction is particularly suited to probe the spectroscopy (i.e. energy, J π and C 2 S p spectroscopic factors) of low-lying states of the residual proton-rich 33 Cl nucleus by comparing measured differential cross section angular distributions with the predictions of theoretical calculations [26,27]. However, while such theoretical predictions reached a level of good consistency, from the experimental point of view several ambiguities are still affecting the spectroscopy of 33 Cl low lying states, concerning especially J π assignments and C 2 S p spectroscopic factors [28]. Such ambiguities arise especially from the presence of contaminants in the observed reaction products spectra that make it difficult to extract the differential cross section angular distributions in wide angular regions [29]. To improve previous studies of the 32 S( 3 He,d) 33 Cl reactions [30,31], an essential ingredient is therefore to reduce possible sources of contaminants produced by undesired elements in the target and, additionally, to improve energy and angular resolution to more effectively disentangle the background from the signals to measure. Finally, a reliable measurement of the absolute cross-section scale is mandatory for a reasonable estimate of the C 2 S p factors.

Experimental setup
The experiment has been performed at the INFN-Laboratori Nazionali di Legnaro (LNL), Italy. A 3 He ++ beam was delivered by the CN accelerator at an energy of 9.8 MeV on a 53 µg/cm 2 nat Zn 32 S target, 99% enriched in 32 S. The nat Zn 32 S powder is evaporated by electron beam technique on a thin substrate of carbon (≈ 15 µg/cm 2 ) to reduce contaminant reactions. The target was installed in the center of a circular scattering chamber, shown in Figure 1. A rotating plate hosted two silicon telescopes, each constituted by an ORTEC surface barrier silicon followed by a high resolution Hamamatsu PIN diode, and an OSCAR telescope [25], placed on the opposite side of the beam line as shown in Figure 1. Both detectors have 2 detection stages to enable the particle identification via the ∆E-E technique. The OSCAR telescope was composed by a thin (≈ 20 µm) 16-strips Single-Side Silicon Strip Detector (SSSSD) manufactured by Micron Semiconductors used as the first detection stage, and 16 individual silicon PIN diode pads (300 µm thick) manufactured by Hamamatsu used as the second detection stage. Each strip of the first detection stage is a 3.125 mm wide vertical silicon strip, while the silicon pads used as the second detection stage have an active area of 1 cm 2 and a ceramic frame of 1.4 mm and 3.2 mm on the bottom side. Each pad is installed on an electronic board containing plug-in pre-amplifiers, as discussed in [25] more in detail, while for the first detection stage we used a dedicated 16 channels pre-amplifier which lies below the rotating plate in Figure 1.
To determine the angular position of each detector with respect to the beam line we performed a dedicated measurement using an optical alignment telescope. Each of the measured positions is determined with an accuracy better than 0.2 • . The optical alignment procedure allowed us also to benchmark the geometry information of the SSSSD detector claimed by the manufacturer. In the OSCAR hodoscope, the correlations between the first and second detection stages can be used to constrain with high precision the impact point on the surface of the detector and therefore the emission angle of the particle emitted from the target. We define 64 individual pseudo-telescopes, each identified by the correspondence of a particular strip of the SSSSD with a pad in the second detection stage. This method allows to have 64 detection telescopes using only 32 individual electronics channels. The detection angle associated to each pseudo-telescope is defined as the geometrical center of the corresponding strip-pad correspondence. These are calculated with a Monte Carlo tool, starting from the absolute position of the OSCAR hodoscope measured with the optical telescope. In the procedure we generate random directions of incidence in the OSCAR telescope and then calculate the azimuthal and polar angle center of each pseudotelescope from the corresponding distributions. Effective solid angles of each pseudo-telescope were benchmarked by means of a dedicated Monte Carlo tool as described in Ref. [25]. During the experiment, we used two different position of OSCAR in order to cover a larger region of detection angles, improved with respect to the state-of-the-art literature [30,31]. In the first of the two geometry positions, OSCAR covered the laboratory angles from about θ = 20 • to about θ = 40 • , while in the second the laboratory angles from θ = 40 • to θ = 60 • are covered. The overall coverage of the cumulative data is shown in Figure 3 in a deuteron energy vs θ plot.

Status of the data analysis
Particles in the OSCAR hodoscope are reconstructed by correlating strips in the first detection stage with pads in the second detection stage, according to the algorithm described in Ref. [25]. The energy deposited in the detector is calibrated independently for the first and the second stage by using the elastic scattering 3  quality of the energy calibration while merging data collected at different times during the experiment, we have produced a correction factor for each pseudo-telescope and each different run of the experiment. This procedure allows to maximize the statistics on the recorded spectra by maintaining an excellent energy resolution. The particle identification is obtained by means of the ∆E-E identification technique, using the signals produced by the first (E strip ) and the second (E pad ) detection stages within a pseudotelescope. Figure 2 shows a typical ∆E-E plot of a pseudo-telescope. Loci corresponding to protons, deuterons, 3 He and 4 He are clearly identified, as indicated by labels. We use a graphical cut procedure to unambiguously identify particles. Bumps lying on the observed ∆E-E loci are produced mainly by the occurrence of binary nuclear processes, such as scattering or binary reactions. Particularly interesting is the case of the deuteron locus, for which several bumps are well visible with a small background level, indicating the emission of deuterons from ( 3 He,d) transfer reactions. In the following analysis process, we will focus exclusively on well-identified deuterons.
In order to properly disentangle various ( 3 He,d) reactions induced on our target, we studied the kinematic lines associated to well-reconstructed deuterons in OSCAR. This is an advantage, with respect to previous investigations, introduced by the use of a new generation hodoscope, where many detection angles are achieved within the same geometry configuration. Figure 3 shows the deuteron kinetic energy as a function of the detection angle (θ) deduced with the procedure described above. The figure is produced by summing together data from 128 pseudotelescopes, 64 for each of the two geometry configuration used during the experiment, covering The angular range shown in Figure 3 is segmented into angular regions according to the recorded statistics and to minimize the θ overlap between pseudo-telescopes of contiguous regions. We obtain over 30 separated angular regions. Within each region, we performed a multi-gaussian fit of the recorded peaks in the deuteron E kin spectrum, according to the findings of Figure 3. In order to achieve a good consistency of the fit, we perform a simultaneous fit of data by using a function which contains the sum of all the gaussians. A typical example of such fits is shown in Figure 4 Figure 3. The 16 O( 3 He,d) 17 F(0.495) peak is strongly suppressed at this angle, and its contribution to the high energy tail of the 32 S( 3 He,d) 33 Cl(2.686) is negligible. The 32 S( 3 He,d) 33 Cl(0.810) is partially overlapped with the prominent 12 C( 3 He,d) 13 Cl(g.s.) peak, because of the kinematics, but, thanks to the excellent resolution of our apparatus, we are able to resolve both yields. The latter is particularly important at all angles, representing the major contaminant in our investigation, and making it extremely difficult to extract the yield of the 32 S( 3 He,d) 33 Cl(0.810) reaction in the angular region 25 • ≤ θ ≤ 35 • . However, at angles greater than ≈ 40 • the separation in energy of these reactions is sufficiently large to allow an easy separation of the corresponding yields.
Finally, to extract the absolute cross-section of each of the measured 32 S( 3 He,d) 33 Cl reactions, we reduced indeterminations arising from the target thickness and the beam charge integration by internal normalization to the 3 He + Zn Rutherford scattering. The latter appears as a prominent peak in the 3 He kinetic energy spectra, well visible at all angles.

Conclusions and Perspectives
To try to solve the ambiguities in the experimental knowledge of low-lying states in the protonrich 33 Cl nucleus, we have performed an investigation of the 32 S( 3 He,d) 33 Cl one-proton transfer reaction by using the new generation hodoscope OSCAR. The combination of the excellent