A new low-energy radioactive beam line for nuclear astrophysics studies in China

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Abstract

A gas-target system has been newly developed and installed at a Radioactive Ion Beam Line in Lanzhou (RIBLL), which makes RIBLL capable of delivering intense, low-energy RIBs for nuclear astrophysics studies. A 1.7×104-pps intensity 22Na RI beam has been achieved in a commissioning run. With further improvements, a 105-pps intensity of some RI beams with mass A<30 (close to the line of β-stability) can be expected in the near future. Some detection equipment including Germanium and Silicon arrays (or balls) available (or to be constructed) at IMP are briefly introduced. A new low-energy radioactive beam line in China is now ready for nuclear astrophysics studies.

Introduction

For a given stellar temperature T, nuclear reactions take place in an energy window around the effective burning energy E0. E0 is the so-called Gamow energy [1], and can be calculated by E0=1.22(Z12Z22μT62)1/3 (E0 in units of keV). Here, μ is the reduced mass in unit of amu, and temperature is in units of 106 K (T6). Z1 and Z2 are the charges of target and projectile. For the explosive hydrogen and helium burning in stars, the temperature can be very high, about several GK (T9). The corresponding thermonuclear reactions occur at Gamow energies around several MeV. To understand the nucleosynthesis of stellar explosions, it is necessary to study reactions involving radioactive nuclei. Radioactive targets can be used for cross-section measurements, but they are restricted to nuclei having fairly long half-lives, e.g., for 7Be, 22Na, 26Al, or 44Ti. Shorter half-lives require the use of radioactive beams [2]. Therefore, low-energy radioactive ion beams (RIBs) at energies of several MeV/nucleon are very useful for nuclear astrophysics studies [3].

A fragment separator, the Radioactive Ion Beam Line in Lanzhou (RIBLL) [4], [5], has been constructed at the Heavy Ion Research Facility of Lanzhou (HIRLF) [6], [7] and operated since 1998. RIBLL was designed as a double-achromatic anti-symmetry separator. Fig. 1 shows a schematic view of RIBLL, which has three focal points (T0, T1, and T2) and two focal planes (C1, C2). The horizontal momentum dispersion (Δx/Δp/p) is about 20 mm/% at C1&C2. RIBLL is similar to those widely used Projectile-Fragmentation (PF) type RIB separators, such as LISE at GANIL [8], A1900 at NSCL [9], RIPS at RIKEN [10], and FRS at GSI [11].

In the past 15 years, the operation of RIBLL has been based on coupling two cyclotrons together, a Sector Focus Cyclotron (SFC, K=69) for low-energy ions (10MeV/nucleon 12C6+) and a Separate Sector Cyclotron (SSC, K=450) for intermediate-energy ions (100MeV/nucleon 12C6+). The RIB production is based on the projectile-fragmentation reaction mechanism. The research scope mainly covers six aspects: (i) measurement of interaction cross-sections for some unstable nuclei [12], [13], [14], [15]; (ii) isospin effect on fragmentation reactions induced by intermediate energy heavy ions [16], [17], [18]; (iii) proton halo studies [19], [20]; (iv) β-delayed neutron decay studies [21], [22], [23]; (v) two-proton emission studies [24], [25], [26]; (vi) elastic scattering of proton-rich nuclei on heavy ions [27], [28].

Recently, a low-energy nuclear astrophysics study [29] has been carried out at RIBLL with two coupled cyclotrons. A primary beam of 20Ne10+ was accelerated up to 69.5 MeV/nucleon and bombarded a 4094-μm 9Be primary target. A secondary beam of 17F9+, which was produced via a projectile-fragmentation reaction mechanism, was then separated and transported by RIBLL to the secondary target chamber. The energy of 17F beam was reduced to the required 4.2 MeV/nucleon on the secondary target through several thick degraders (a 313μm aluminum plate at C1, and a 323μm silicon plate with a 294μm aluminum plate at T1). The intensity of the 20Ne primary beam was about 2×1011 pps, which is at present the best performance achieved on this beam line. In this way, the intensity obtained for the 17F beams was only about 1×103 pps, which is far below what is needed in nuclear astrophysics studies. In a previous beam test [4] with two coupled cyclotrons, more than 3×105 pps intensity of this beam was achieved at an energy of 50 MeV/nucleon. The huge decrease is mainly ascribed to those thick degraders used for reducing the beam energy. This fact demonstrates that it is impossible to obtain low-energy and high-intensity (in >104pps order) RIBs via the projectile-fragmentation approach at RIBLL.

In order to obtain low-energy (2–4 MeV/nucleon) and high-intensity RIBs on the secondary target, we have developed a gas-target system for secondary beam production. The primary beam accelerated by the SFC is directly transported to RIBLL through a beam line constructed a few years ago. The low-energy primary beam bombards the gas target, and the secondary beam produced is subsequently separated and transported to the secondary target chamber by RIBLL. Actually, this technique was proposed over 30 years ago [30], [31]. The species of interest are produced “in-flight” by a transfer reaction (or fusion evaporation reaction) between primary beam particles and nuclei in a gas or a thin solid target. This new setup makes RIBLL similar to those existing facilities, such as, CNS Radioactive Ion Beam separator (CRIB) of the University of Tokyo at RIKEN [32], [33], [34] and Momentum Achromat Recoil Spectrometer (MARS) at Texas A&M University [35], [36], [37], [38]. A comparison of main specifications of these facilities is made as listed in Table 1.

The work presented in this paper has mainly demonstrated some new features of RIBLL and some detection equipment available (or to be constructed) at IMP. In Section 2 we describe the production gas-target system, including details on the gas cell, gas flow system, and cooling apparatus. The result of the commissioning run for the 22Na RIB production is reported in Section 3. Germanium and Silicon detector arrays (or balls) are briefly introduced in 4 , 5 Si telescope. Finally, the outlook of this low-energy beam line is discussed in Section 6.

Section snippets

Gas-target system

Recently, a gas-target system has been installed at T0 of RIBLL as shown in Fig. 1. A photograph of the small gas-target chamber is shown in Fig. 2(a). The basic design is based on that used at CRIB [34]. The only difference is that we made it with brass (melting point about 930–970 °C) instead of aluminum alloy (melting point about 610–650 °C) used at CRIB. The gas is confined in a cylindrical cell (ϕ30mm, 80 mm long) with two 2.5μm-thick Havar windows [39]. The Havar foils were tightly pressed

Result of a commissioning run

The experiment was carried out at HIRFL. A primary beam of 22Ne7+ was accelerated up to 6.18 MeV/nucleon by the SFC, and the intensity was about 2.6×1011 pps after a small collimator (φ8mm) installed at the entrance of RIBLL. The primary beam bombarded the Alcohol-cooled hydrogen gas target. The thickness of hydrogen gas (99.99% purity) was about 0.36 mg/cm2 at 2 °C with 0.5 atm pressure. A secondary beam of 22Na was produced via the 1H(22Ne,22Na)n reaction. The secondary beam was separated,

γ-Ray arrays

A Clover array has already been assembled at IMP for several experiments [49]. In addition, a CsI crystal ball and a Clover-HPGe ball are being constructed now. With these arrays, we are able to detect the γ rays from those nuclear reactions, e.g., resonant inelastic scattering of p(F17,F17)pγ [50] or other in-beam γ spectroscopy of nuclear astrophysics relevance. For instance, the properties (excitation energy and spin-parity) of those proton-unbound states in some nuclei have been studied

Si telescope

At present, some Micron silicon detectors [60] are available at IMP, which can be combined into several sets of ΔEE telescopes. Here, the ΔE detector is a position sensitive detector W1- or BB7-type with thickness ranging from 65to1000μm, E is the MSX25-type pad detector with thickness ranging from 300to1500μm. In addition, IMP is now committed to developing silicon strip and pad detectors [61], [62], [63]. It will supply a variety of silicon detectors in the near future.

Discussion and outlook

An Alcohol-cooled gas-target system has been developed for low-energy RIBs production at RIBLL recently. An intensity of 1.7×104 pps has been achieved for the 22Na RI beam with a 2.6×1011 pps 22Ne primary beam and a 0.36 mg/cm2 hydrogen gas target. As a reliable expectation, an intensity of about 105-pps order of 22Na RI beam can be achieved by improvement in two aspects: (1) increasing the primary beam intensity from 2.6×1011 pps to about 1×1012 pps (intensity at the exit of SFC is about 20 times

Acknowledgments

We would like to acknowledge the staff of HIRFL for operation of the cyclotron and those of RIBLL for their friendly collaboration. J.J. would like to express appreciation to D. Kahl (CNS, the University of Tokyo) who made helpful comments on the manuscript. This work is financially supported by the National Natural Science Foundation of China (11135005, 11021504, 10975163) and the “100 Persons Project” (BR091104) of Chinese Academy of Sciences.

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