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Novel multi-layer plastic-scintillator-based solid active proton target for inverse-kinematics experiments

We have constructed and tested a novel plastic-scintillator-based solid-state active proton target for use in nuclear spectroscopic studies with nuclear reactions induced by an ion beam in inverse kinematics. The active target system, named Stack Structure Solid organic Scintillator Active Target (S4AT), consists of five layers of plastic scintillators, each with a 1-mm thickness. To determine the reaction point in the thickness direction, we exploit the difference in the energy losses due to the beam particle and the charged reaction product(s) in the scintillator material. S4AT offers the prospect of a relatively thick target while maintaining a good energy resolution. By considering the relative energy loss between different layers, the energy loss due to unreacted beam particles can be eliminated. Such procedure, made possible by the multi-layer structure, is essential to eliminate the effect of unreacted accompanying beam particles, thus enabling its operation at a moderate beam intensity of up to a few Mcps. We evaluated the performance of S4AT by measuring the elastic proton-proton scattering using a 70-MeV proton beam at Cyclotron and Radioisotope Center (CYRIC), Tohoku University.


Introduction
Nuclei beyond the beta-stability line, commonly known as radioactive isotopes or unstable nuclei, are currently at the forefront of nuclear physics research. Over the last few decades, extensive experiments using radioactiveisotope (RI) beams have revealed various exotic and fascinating features of nuclear structure and dynamics hitherto unknown in nuclei on or near the βstability line. Of particular interest are the formation of neutron-halo structure [1] and evolution of nuclear magic numbers [2,3,4] in neutron-rich nuclei.
While the exact mechanisms for both phenomena have remained unclear, it seems that the tensor interactions may play an important role [5,6]. The observations of possible effect of the tensor interactions in 16 O via (p,d) [7] and (p,dp) [8] reactions on 16  The unstable nuclei, with long isotopic chains, provide the ideal platforms for systematic studies using nuclear reactions. The increasing availability of a wide range of highly-or moderately-intense RI beam species, and developments of more complex and sophisticated detector systems such as multi-strip silicon arrays [9,10] and active gaseous target detector systems [11,12,13,14,15] have enhanced the prospect for spectroscopic studies of unstable nuclei with nuclear reactions. One of the most versatile experimental methods via nuclear reactions is the missing-mass spectroscopy. In this method, the momentum/momenta of the reaction product(s), usually light ion(s), is/are measured. The momentum information obtained is then used to reconstruct the excitation energy of the residual nucleus right after the reaction, and to determine physical observables such as differential cross sections. To ensure sufficient separation for the ground and different excited states in the residual nucleus, it is important to attain good momentum resolution. For experiments with stable beams and stable target nuclei, a good momentum resolution is usually fulfilled by using a thin target.
The same prescription, however, is seldom applicable to experiments with RI beams due to their limited beam intensities. Hence, to increase luminosity and thereby improve the experimental feasibility, it is essential to increase the target thickness whenever possible. The use of a thicker target, however, may result in the degradation of the momentum resolution due to the uncertainty in the depth of reaction vertex.
Here, we report on the development of a novel multi-layer solid-state active proton target, named Stack Structure Solid organic Scintillator Active Target (S 4 AT). The device consists of ordinary commercial plastic scintillators, and is therefore highly affordable and flexible; such advantages have already been demonstrated by the recently developed S 4 neutron detector [16]. S 4 AT offers a versatile alternative proton (or deuteron) target for spectroscopic studies with radioactive isotope (RI) beams using nuclear reactions, especially reactions that result in more than one charged particle being ejected, in inverse kinematics.
Because the total energy loss of charged particles in the scintillator material differs significantly before and after such reactions, one can determine the reaction point in the thickness direction by measuring the energy-loss distribution across different layers of the active target. This reaction-depth determination capability allows the use of a relatively thick target while maintaining a relatively good energy resolution. The multi-layer structure is of importance because it provides a means to subtract the energy loss due to unreacted beam particles through consideration of the energy-loss difference between adjacent layers.
Such feature is essential to eliminate the effect of accompanying unreacted beam (pile-up) particles. These depth-sensitive and pile-up elimination capabilities, together with the short decay time of the plastic scintillator, enable its operation at a moderate beam intensity of up to a few Mcps. To assess the performance of S 4 AT, we have conducted a test experiment using elastic proton scattering with a 70-MeV proton beam at Cyclotron and Radioisotope Center (CYRIC), Tohoku University.
The article is organized as follows. In Section 2, we describe the basic principle for vertex determination and the prototype system. The test experiment is presented in Section 3. The data analysis and results are presented in Section 4.
Finally, a summary and the future prospects are given in Section 5.

Principle for vertex determination and prototype
The present S 4 AT prototype was designed so as to achieve depth resolution below 0.29 mm (in sigma), or 1/ √ 12 mm, for the reaction vertex. This depth resolution is necessary to meet our requirement for excitation energy resolutions of about 1 MeV in p( 6 He,d) and p( 6 Li,d) reaction measurements in inverse kinematics using 400 -800 MeV/u secondary beams at GSI Helmholtzzentrum fr Schwerionenforschung GmbH, Darmstadt. The goal of the experiment is to observe possible different effects of tensor interactions in the ground states of 6 He and 6 Li [17]. In this section, we describe the principle for vertex determination and the constructed prototype.

Principle for vertex determination
The left diagram of Fig. 1 shows a conceptual drawing of a three-layer plasticscintillator active-target system. Here, we consider two identical charged beam particles, denoted by A and B, with the same kinetic energy. The particle A reacts with the detector-target nucleus to produce two charged particles C and  To demonstrate the working principle of S 4 AT, we consider the p( 6 He,d) reaction induced by a 400-MeV/u 6 He beam, and a layer thickness of 1 mm for S 4 AT. In this reaction, the incident 6 He beam particle (particle A) is stripped of a neutron by a target proton in S 4 AT, leaving the residual ion 5 He (particle C) in its ground or excited state. The neutron and the proton stick together to form a deuteron (particle D). Since 5 He is particle unbound, it decays almost immediately into a neutron and an α particle. Compared to charged particles, As will be shown in Section 4, for single-hit events, the total energy loss (or even the energy loss in individual layers if the energy resolution is sufficiently good) has a one-to-one correspondence with the depth of reaction.
The layer thickness of 1 mm was decided based on a simple evaluation using a Monte Carlo simulation, and after taking into account the energy losses and the energy resolution of the plastic scintillators.
The one-to-one correspondence between energy loss and reaction depth may become obscure in the presence of multi-hit events. The probability for multihit events increases with increasing beam intensity. The admixture of the B-like particles in the reaction channel of interest will alter the energy-loss distributions, and may in some cases, degrade the energy resolution and undermine its performance as an active target. Such an effect of non-reacted particle(s) can be eliminated by considering the energy-loss difference between adjacent layers. The depth information is reflected in the correlation between energy-loss differences of two pairs of adjacent-layer combinations. This method will be discussed in Section 4.3.

S 4 AT prototype
We have constructed a prototype S 4 AT system. Figure

Experiment
The constructed S 4 AT prototype was tested using elastic proton scattering induced by a 70-MeV proton beam at the 41st beam line of CYRIC, Tohoku University. This reaction was selected because of the similarity between the energy losses of the proton beam and the scattered protons with those of the charged particles in the p( 6 He,d) reaction with a 400-MeV/u 6 He beam, as well as the relatively large cross section and high luminosity. Fig. 3 shows a schematic layout of the experimental setup. The S 4 AT system was mounted on a target ladder with the 50×30-mm 2 surface perpendicular to the beam, and placed at the centre of a scattering chamber. We define the axis along the beam direction as the z axis; the x axis is defined as shown in Fig  shaping amplifiers before being fed to 32-channel peak-sensing analog-to-digital converters (MADC-32).
For S 4 AT, the output signals from MAPMT were sent to a photomultiplier amplifier. One of the two outputs from each channel was sent through a cable delay before being fed into a 32-channel charge-to-digital converter (CAEN V792) module; the other output was fed into a leading-edge discriminator to generate a timing signal, which was mainly used to determine the beam intensity.
Measurements were performed using proton beams with intensities ranging from about 75 kcps to 3.6 Mcps. The beam intensities were determined combining the scaler information for S 4 AT and the telescopes, as well as the relative event numbers from measurements with different beam intensities. For the measurements with the CH 2 target, a beam intensity of about 0.8 nA was used.
Data acquisition was performed with the logic OR signal of the SSD signals as trigger using the software package babirlDAQ [18].

Data analysis and results
The data analyses were performed using the object-oriented data analysis framework, ROOT [19].  The small clusters distributed along the proton and deuteron loci correspond to the ground and excited states in 12 C and 11 C. These states, together with the proton-proton elastic scattering events measured at different angles, were used for the energy calibration of the SSD and CsI(Tl) detectors. Fig. 4(b) shows the correlation between the total energy losses measured by the left telescope (E L ) and the summed energy loss in the left and right telescopes ( The dense locus corresponds to proton-proton elastic scattering events. The energy resolution of the telescopes for proton scattering was evaluated using the observed discrete states in 12 C as well as the proton elastic scattering events during the measurement with the CH 2 target. Since the energy resolu- From comparison between the simulations and the experimental data, the beamenergy spread was found to be less than 200 keV (in sigma). The relative energy resolutions of the CsI(Tl) detectors thus determined consist of three terms [21], namely the stochastic, electronic-noise and constant terms as shown below:  To determine the depth of the reaction vertex, we apply three methods using the following information: (i) the total energy loss of the scattered protons in the telescopes, (ii) the total energy loss of charged particles in S 4 AT, and (iii) energy-loss differences between different layers of S 4 AT. In principle, E L + E R (≡ E tot ) should have a one-to-one correspondence with the reaction depth. However, as can be seen in Fig. 5(b), the E tot distribution is skewed especially for protons with larger scattering angles (smaller E L ) due to their larger energy spread. This skewing effect was estimated using Monte Carlo simulations, and E tot was corrected for each E L value. The corrected distribution (E corr tot ) is shown in Fig. 6(a). In this way, we obtained a linear conversion function between the total energy of the scattered protons and the reaction depth. The reconstructed depth distribution is shown in Fig. 6(b).

Vertex determination with SSD-CsI(Tl) telescope
For comparison, the depth distribution reconstructed from Monte Carlo simulation data is also shown (the red-dotted histogram). Overall, the simulation reproduced the experimental data very well.
It is important to note that the resolution of the reaction depth determined with the telescopes remained almost the same up to 3.6-Mcps proton beam, as shown in Fig. 7. The detected events reacted in L1, which include the events at z < 0 mm, are about 16% less compared to other layers, due likely to reduced acceptance of the telescopes for protons with large scattering angles. These missing events correspond to about 2% of the expected total pp-elastic events assuming that the elastic scattering cross section remains constant in all layers.

Vertex determination with S 4 AT: Total energy-loss method
The depth of the reaction vertex can be determined by simply considering cumulative (total) energy loss of charged particles in S 4 AT. This is equivalent to replacing the five layers by a single 5-mm-thick plastic scintillator. In this method, the total energy loss of the proton beam before reaction and the two protons (p L and p R ) after reaction in S 4 AT is measured. The total energy loss in S 4 AT decreases with increasing z r because p L and p R , which have lower energies, penetrate through thinner plastic scintillator material. Fig. 8(a) shows the correlation between total energy loss (denoted by Q ATsum ) and the reaction depth determined using the SSD-CsI(Tl) telescopes as described in Section 4.1.
Here, only events within the 'pp-elastic gates' have been selected. Events due to pile-up, which manifest in the form of energy-loss shifts as shown by the arrows, in S 4 AT are clearly observed. The energy resolution for 1-MeV energy loss of a proton beam in a layer of S 4 AT was about 18% at 75-kcps beam intensity. As will be discussed in Section 4.4, the reaction depth resolution depends on the energy resolution of S 4 AT.
To reconstruct the reaction depth from Q ATsum , we derived a conversion formula using the single-hit events in Fig. 8(a). The resulting reaction depth (denoted by z ATQsum r ) is plotted against z r determined by the telescopes (denoted by z Tel r ) in Fig. 8(b). For comparison, similar plots for the measurement with 740-kcps proton beam are shown in Figs. 8(c) and (d). The energy resolution deteriorates due to the increased pile-up events. It is important to note that although the single-hit, double-hit and even the triple-hit loci are still distinguishable, without the depth information from the telescope, it will be difficult  Fig. 8(e) with those of (c) and (a), clear reduction in the pulse heights of all anode signals. Such reduction is due to possible increased electron current which results in a drop in the interstage voltages for the dynodes in the MAPMTs, and consequently reduced collection efficiency and secondary electron emission. Although the gain-quenching effect can be addressed by modifying the last few stages of the MAPMTs to allow additional power supply boosters, the pile-up events will represent the major challenge for this method at a high-counting rate. In the next section, we demonstrate a novel method to address this issue.

Vertex determination with S 4 AT: Energy-loss difference method
As mentioned earlier, the presence of nonreacted beam particle(s) in a reaction event results in a shift in the measured energy loss. For intermediateenergy beam passing through thin layers of plastic scintillators, the energy loss of a beam particle in each layer is almost constant. Hence, this energy-loss shift, i.e. the effect of pile-up, can be eliminated by taking the energy-loss difference between adjacent layers. The simplest method is to consider energy loss in every single layer. Fig. 9(a) shows the scatter plot for the energy-loss difference between L2 and L3 (∆E L3 − ∆E L2 ) against the energy-loss difference between L1 and L2 (∆E L2 − ∆E L1 ) for the measurement with 75-kcps proton beam.
Here, we have selected only coincident protons using the telescopes. Different points on the sides of the triangular locus correspond to different reaction depth in layers L1 -L3. The arrows indicate the direction of the depth position.
The dense elliptical events correspond to events reacted in L4 and L5. Similar correlations can be obtained by considering other combinations of energy-loss difference, and used to determine the reaction depth. This single-layer energyloss-difference method, however, is subject to inherent uncertainties around the borders of adjacent layers, which correspond to the corners of the triangular locus in Fig. 9(a). For comparison, we plot the reaction depth reconstructed  from this method, denoted as z AT−SLQrel r against z Tel r in Fig. 9(b). Figs. 9(c) and (d) show similar plots as (a) and (b) but for the measurement with the 740-kcps beam. The loci in Fig. 9(c) are almost identical to those in Fig. 9(a) We applied this method to measurements with various beam intensities. The performance of S 4 AT is evaluated in terms of the depth position resolution and tracking efficiency. To evaluate the depth position resolution, we consider the difference between the reaction depth determined using the telescopes and the one with S 4 AT: where AT is "ATQsum" or "ATQrel;2". The ∆z AT distributions are shown in Figs. 12(a) and (b). The root-mean-square width of the ∆z AT distribution around 0, denoted as σ AT z , is taken as the effective depth position resolution, which is attributed to the intrinsic energy resolutions of the plastic scintillators and the telescopes. The tracking efficiency is defined as where N (|∆z AT | ≤ 2.33σ AT z ) represents the number of events with ∆z AT within 2.33σ AT z or 98% coincidence level, and N (−1 < z Tel is denoted by σ ATQ;L i . As mentioned earlier, σ ATQ;L i was about 18% at 75-kcps beam intensity in the present work. This relatively poor intrinsic energy resolution was due to light leakage into neighbouring anode strips, which resulted in a relatively low light collection efficiency (about 60%) in one anode strip. Further reduction in σ ATQ;L i is anticipated in future experiments by adding back the output signals from neighbouring anodes.

Summary and future prospects
We have constructed and tested a novel prototype solid-state active proton target, named Stack Structure Solid organic Scintillator Active Target (S 4 AT), for use in nuclear spectroscopic studies with nuclear reactions in inverse kinematics. The solid active target consists of five layers of plastic scintillators, each with a 1-mm thickness. S 4 AT offers the capability to determine the reaction depth through exploitation of the difference between the energy losses of a charged beam particle and charged reaction products in the scintillator material. By considering the relative energy loss between different layers, the energy loss due to unreacted beam particles can be eliminated. The ability to eliminate pile-up effects enables its operation at a moderate beam intensity of up to a few Mcps. To evaluate its performance, we have performed an elastic proton-proton measurement using a 70-MeV proton beam at Cyclotron and Radioisotope Center (CYRIC), Tohoku University. The depth resolution achieved was below 0.3 mm (in sigma) at 3.6-Mcps proton beam intensity.
One of the two remaining issues is the gain shift, which will be the most critical issue for operation at higher beam intensity. To address this issue, we plan to introduce additional power supply boosters to the last few stages of the MAPMT. The other issue is related to its performance with different types of beam. The present test experiment was performed using a proton beam from a cyclotron. It will be useful and important also to perform a test experiment using a beam from a synchrotron accelerator, because of the difference in the time structure of the beam, i.e. the pile-up frequency.
The present system offers the prospect of a relatively thick target while maintaining a good energy resolution. It is worth noting that by replacing the plastic scintillators with the deuterated ones such as the BC436 [22], one can also construct a solid-state active deuteron target system.