A detection system for charged-particle decay studies with a continuous-implantation method

https://doi.org/10.1016/j.nima.2015.09.039Get rights and content

Abstract

A new detection system with high detection efficiency and low detection threshold has been developed for charged-particle decay studies, including β-delayed proton, α decay or direct proton emission from proton-rich nuclei. The performance was evaluated by using the β-delayed proton emitter 24Si produced by projectile fragmentation at the First Radioactive Ion Beam Line in Lanzhou. Under a continuous-beam mode, the isotopes of interest were implanted into two double-sided silicon strip detectors, where the subsequent decays were measured and correlated to the preceding implantations by using position and time information. The system allows us to measure protons with energies down to about 200 keV without obvious β background in the proton spectrum. Further application of the detection system can be extended to the measurements of β-delayed proton decay and the direct proton emission of more exotic proton-rich nuclei.

Introduction

A variety of exotic decay modes have been observed for proton-rich nuclei far from stability, such as β-delayed particle emission and direct particle emission. The β-delayed proton spectroscopy and proton radioactivity studies have proved to be powerful tools to investigate the nuclear properties and to obtain the structure information close to and beyond the drip-line. The resonant proton capture reaction rates of astrophysical interest can be evaluated based on information about the properties of states close to the proton-separation threshold [1], [2], [3]. Observations of decays from those states and direct proton emissions from ground states both require a low detection threshold of energy and a high detection efficiency. Various experimental setups for β-decay spectroscopic studies and proton radioactivity studies have been developed and accordingly, significant progress has been made in the studies of the properties of exotic nuclei [4], [5], [6], [8], [7], [9], [10], [11], [12].

The TZ=2 proton-rich nucleus 24Si has been measured with various detection methods, which provides a good opportunity to compare their advantages and disadvantages. 24Si was first observed through helium-jet techniques by Äystö et al. [13] in 1979, followed by two similar experiments to determine the half-life of 24Si and the excitation energy of isobaric analog state in 24Al [14], [15], [16]. The results provide a good test of the validity of the isobaric multiplet mass equation in spite of the low statistics and the high contamination from neighboring nuclei.

At the end of the last century, Czajkowski et al. [17], [18], [19] performed the first spectroscopic study of 24Si by implanting the projectile fragments into a 300 μm thick unsegmented silicon detector. Several new proton peaks were observed and the half-life of 24Si was determined with a small uncertainty. However, a major problem for this implantation method is that the β particle energy-loss yields a high background on the low-energy part of the measured proton spectrum and, consequently, the identification of proton groups is affected severely.

Later, Banerjee et al. [20], [21] overcame this β pile-up problem by implanting the projectile fragments into an aluminum foil and a gas-ΔE, silicon E detector telescope was placed alongside the foil for delayed proton measurement. The β-delayed proton spectrum of 24Si nearly free from β background was obtained by using a two-dimensional plot of ΔEE to separate the protons from the β particles. But the disadvantages of this ΔEE method are that the energy straggling of protons emitted from the foil causes the peak broadening of proton spectrum especially for the low-energy part and moreover, the absolute decay branching ratios cannot be determined.

Recently, Ichikawa et al. [22], [23] modified the ΔEE method by increasing three more telescopes to cover larger solid-angle and combining the γ and proton measurements, as well. A plastic scintillator was used as the beam stopper when the γ-ray measurement was carried out. The absolute branching ratios to the bound states can be deduced from the extra γ-ray measurement, thus the absolute branching ratios to the unbound states can be normalized. The entire decay scheme of 24Si was reconstructed with this upgraded ΔEE method. However, the proton measurements and the γ-ray measurements could not be performed simultaneously, and all the branching ratios for transitions to the unbound states was renormalized based on the γ-transition results, instead of being measured directly. Until very recently, a γ-transition ratio datum was replaced by the new measurement, which led to a reevaluation of all the results involved with this γ-transition ratio parameter, including the branching ratios, log ft values, B(GT) results and even the conclusions about the Thomas–Ehrman shift [24].

For all the above-mentioned experimental methods, it is difficult to measure the low-energy protons for the reason that the resolution of low-energy proton peaks is affected either by the β pile-up effect in the implantation method or by the energy straggling effect in the ΔEE method. For the ΔEE method, there exists an extra paradox that thin stopper foils cannot stop enough ions whereas thick foils will cause a severer energy straggling effect for protons. It should also be noted that all the above-mentioned experiments were carried out under a pulsed-beam mode, i.e. the beam is pulsed on for a period and then the decay measurement is performed during the beam-off period. Therefore the decay events during the beam-on period are missed, and similarly, the implantation events during the beam-off period are prohibited, too. In consequence, the detection efficiency for decays is considerably decreased and the beam utilization is severely limited, causing significant loss of statistics. Recently, more experiments for decay studies have been performed under the continuous-beam mode instead of the pulsed-beam mode in order to achieve a higher detection efficiency [25], [26], [27].

In comparison with the ΔEE method, the implantation method has two advantages: First, there are no dead layers for the protons emitted inside the detector so that any energy-loss of protons can be avoided before they enter the active area of the detector. Second, the absolute decay branching ratios can be estimated directly from the numbers of the implanted ions and the protons from decay with high precision [5]. Hereby, we have developed a new detection system on the basis of our previous experiments with implantation method [28] and the experiments with complete-kinematics measurements [29], [30], [31]. The implantation silicon detector was upgraded to a thin and segmented silicon detector to minimize the β pile-up effect and also to be employed under a continuous-beam mode.

Section snippets

Description of the setup

The schematic layout of the detection setup is presented in Fig. 1. A 149 μm thick double-sided silicon strip detector (DSSD1, W1-type from Micron Semiconductor Ltd. [32]) is used to stop the isotopes of interest, which also serves as a β-delayed proton decay detector. A 66 μm thick double-sided silicon strip detector (DSSD2, W1-type) playing a similar role as DSSD1 is mounted only 10 mm downstream from the DSSD1. This thinner DSSD is aimed at detecting low-energy protons because the β particles

In-beam tests

The performance of the detection system was tested in-beam using the β-delayed proton-emitter 24Si at the First Radioactive Ion Beam Line in Lanzhou (RIBLL1) [34]. The K450 separate sector cyclotron (SSC) provided a 75.8 MeV/u primary beam of 28Si with an intensity of ~ 37 enA (~ 2.6 pnA). The secondary beam was produced via projectile fragmentation of a 28Si beam impinging on a 1980 μm thick 9Be target. The ions of 24Si in the secondary beam were separated and purified by the combined BρΔEBρ

Results

The β-delayed proton decay events should meet the requirement of picking up a signal above the Philips 7106 threshold (typically 100 keV) in the two DSSDs while rejecting the coincident signal in the ΔE-TOF gate. In order to suppress the noise and background disturbance, the energy difference between each decay signal from the junction side strips and that from the ohmic side strips is limited within ±10% as well as no more than ±300keV [5]. The DSSD xy pixel position information is also used

Conclusion

A new decay detection system with an implantation method has been built and commissioned in an experiment of β-delayed proton decay of 24Si under a continuous-beam mode. The setup makes it possible to perform an accurate identification of the implanted nuclei and the subsequent decays by means of energy, time and position measurement. Though the intensity of 28Si primary beam was much lower and the collection time of our experiment was much less than those in the previous experiments of 24Si, a

Acknowledgment

We acknowledge the continuous effort of the HIRFL-RIBLL1 operators for providing good-quality beams, ensuring compatibility of the electronics. This work is supported by the National Basic Research Program of China under Grant No. 2013CB834404, and the National Natural Science Foundation of China under Grants No. 11375268, No. 11475263, No. U1432246, and No. U1432127.

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      In the detection system, two plastic scintillators at the second and fourth focal planes of the RIBLL1 were used for measurements of time of flight (ToF) of fragments. A silicon array [21] coupled with germanium clover detectors at the end of RIBLL1 was used to identify secondary ions on an event-by-event basis and study their decay properties with an implantation-decay correlation. In the silicon array, two silicon detectors in the front were used to measure the energy loss (ΔE) of fragments.

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