The TITAN in-trap decay spectroscopy facility at TRIUMF

doi:10.1016/j.nima.2014.12.118

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Volume 780, 21 April 2015, Pages 91-99

https://doi.org/10.1016/j.nima.2014.12.118 Get rights and content

Abstract

This paper presents an upgraded in-trap decay spectroscopy apparatus which has been developed and constructed for use with TRIUMF׳s Ion Trap for Atomic and Nuclear science (TITAN). This device consists of an open-access electron-beam ion-trap (EBIT), which is surrounded radially by seven low-energy planar Si(Li) detectors. The environment of the EBIT allows for the detection of low-energy photons by providing backing-free storage of the radioactive ions, while guiding charged decay particles away from the trap centre via the strong (up to 6 T) magnetic field. In addition to excellent ion confinement and storage, the EBIT also provides a venue for performing decay spectroscopy on highly charged radioactive ions. Recent technical advancements have been able to provide a significant increase in sensitivity for low-energy photon detection, towards the goal of measuring weak electron-capture branching ratios of the intermediate nuclei in the two-neutrino double beta ( $2 ν β β$ ) decay process. The design, development, and commissioning of this apparatus are presented together with the main physics objectives. The future of the device and experimental technique are discussed.

Introduction

The characterization of radioactive decay via photon detection is a key measurement method and is among the primary experimental techniques currently employed in nuclear physics [1]. With the advancement of rare-isotope beam (RIB) facilities worldwide [2], access to increasingly exotic radioactive nuclei has become possible, allowing for a variety of decay experiments on short- and long-lived nuclei. Modern decay spectroscopy devices employ multiple detection systems for both charged particles and photons to further increase the sensitivity of the experiment, thus allowing for the observation of weak signals [3]. The reduction of photon backgrounds is at the forefront of these efforts, and requires a high level of control over the decay environment which can be provided using ion traps [4].

The concept of observing decays from trapped radioactive nuclei has been employed for years, most notably using magneto-optical traps and Paul traps, where charged particles and daughter recoils are detected to provide direct and indirect information about neutrinos [5], [6], [7], [8], [9], electrons [10], and neutrons [11]. More recently, Penning traps have been considered to provide control over the decay environment [12], [13], [14], [15], [16], [17], [18], [19], [20], where charged particles are guided along strong magnetic-field lines. Therefore, further extension of this concept may be possible for performing high-sensitivity decay-spectroscopy measurements.

Recent evidence that neutrinos have mass has generated a great deal of interest in exotic nuclear decay modes [21], [22]. As a part of these studies, searches for the neutrinoless ( $0 ν$ ) mode of double beta ( $β β$ ) decay is among the most relevant since it violates lepton-number conservation and would establish the neutrino as a Majorana particle [23], [24]. If this decay mode is observed, the effective Majorana mass of the neutrino, $〈 m_{β β} 〉$ , can be deduced from $0 ν β β$ measurements: ${(T_{1 / 2}^{0 ν})}^{- 1} = G^{0 ν} (Q, Z) | M^{0 ν} |^{2} {〈 m_{β β} 〉}^{2}$ where $T_{1 / 2}^{0 ν}$ is the observed half-life of the $0 ν β β$ decay and $G^{0 ν} (Q, Z)$ is the phase-space factor. The term $M^{0 ν}$ is the nuclear matrix element (NME) connecting the initial and final $0^{+}$ states, which results entirely from theoretical calculations. The calculation of $β β - decay$ NMEs is the source of current theoretical efforts and includes several different model descriptions. The accuracy and precision from Eq. (1) is limited by the ability to calculate the NMEs, and any uncertainty in the calculations is directly translated to $〈 m_{β β} 〉$ . Therefore, constraints on these calculations are required from detailed experimental data.

Typically, the NME calculations are benchmarked to two-neutrino ( $2 ν$ ) $β β$ data [25] (a process allowed by the Standard Model) where the decay path proceeds through $1^{+}$ states in the odd–odd intermediate nucleus [26]. Therefore, measurements of the β⁻ and electron-capture (EC) branching ratios of the intermediate nuclei in the $2 ν β β - decay$ process are directly relevant for capturing the nuclear-physics information required in the calculation of $M^{2 ν}$ . The EC transitions are several orders of magnitude weaker than the dominant β⁻ decays from the same parent nucleus, making them difficult to detect. Due to the weak nature of these decay branches, these studies require intense RIBs and low-background, high-sensitivity decay spectroscopy tools [26], [16].

The Isotope Separator and Accelerator (ISAC) facility [27], [28] at TRIUMF in Vancouver, Canada, employs a high-intensity (up to $100 μ A$ ) beam of 500 MeV protons to produce RIBs using the isotope separation on-line (ISOL) technique [2], [29]. ISAC is currently able to provide a wide variety of RIBs through the use of several different production target and ion-source combinations, including the recent use of uranium–carbide (UC_x) targets [30]. Following the in-target production and ionization, the ions are mass separated before being delivered to the experimental hall. The mass-selected, continuous beam of radioactive, singly charged ions (SCIs) is delivered at low energies ( $< 60 keV$ ) to a suite of experimental facilities for both cooled- and stopped-beam experiments [30], where TRIUMF׳s Ion Trap for Atomic and Nuclear Science (TITAN) [31], [32] is located. The TITAN system consists of three ion traps:

1.
a radio-frequency quadrupole (RFQ) linear Paul trap [33], [34] for buffer-gas cooling and bunching the low-energy ion beam,
2.
a 3.7 T, high-precision mass-measurement Penning trap (MPET) [35], and
3.
an electron-beam ion trap (EBIT) which is used to create highly charged ions (HCIs) [38], and for performing decay spectroscopy on trapped radioactive nuclei.

A schematic view of the TITAN facility at TRIUMF-ISAC is shown in Fig. 1, along with the ion path for typical decay-spectroscopy experiments.

The prospect of performing decay spectroscopy with TITAN was first presented in Refs. [13], [16]. In these measurements, a low-energy germanium (LEGe) detector was placed in the EBIT for photon counting, and no electron beam was employed for ion confinement of charge breeding. In this mode of operation, the EBIT effectively serves as a cylindrical Penning trap. The results from these measurements demonstrated that ions could be injected, stored, and extracted from the EBIT for the purpose of decay spectroscopy, however storage times were limited to tens of ms due to losses at the trap center. Additionally, the in-trap losses meant that information regarding the precise location of where the decays were occurring was lost, and thus a determination of the photon detection efficiency was not possible. Since the primary science goal of this apparatus is the characterization of weak decay branches (10⁻³–10⁻⁵), an improvement of the experiment was required, and new techniques were developed.

This paper presents a significant upgrade to the apparatus, and addresses the above deficiencies towards the goal of high-sensitivity in-trap decay spectroscopy. These improvements include a new trapping mechanism, different photon detectors, improved ion-bunch manipulation, a superior data-acquisition system, and improved environmental monitoring and control.

Section snippets

The decay spectroscopy trap

The TITAN EBIT (Fig. 2) is composed of an up-to 500 mA electron gun,² a cold drift-tube assembly which is thermally coupled to a superconducting magnet, and an electron collector. The drift-tube assembly is conically shaped which improves the trapping profile while retaining a large geometric acceptance for the incoming ions. The 8-fold radially segmented central electrode forms the potential where the

Planar Si(Li) detectors

Each of the seven access ports around the EBIT houses a lithium-drifted silicon detector (Si(Li)) (Fig. 6), which has good resolution and high efficiency at low photon energies ( $< 50 keV$ ) [1]. These detectors were chosen over high-purity germanium (HPGe) crystals due to their decreased X-ray escape peak intensity³ and the prospect of performing a high-sensitivity X-ray measurement on ⁷⁶Ge [26].

Environmental effects

The TITAN facility is located roughly 5 m above the floor on a raised platform in the ISAC-I experimental hall. As a result, environmental fluctuations which may affect the sensitivity of the experiment must be continuously monitored. To accomplish this, several diagnostic components are situated in various locations around the experimental setup, including thermocouples, vibration sensors, optical-light sensors, and voltage monitors.

On-line commissioning

The first on-line commissioning with radioactive beam was performed using six Si(Li) detectors, and is reported in Ref. [19]. The goal of this measurement was to characterize and examine the capabilities of the setup through the observation of X-rays resulting from ¹²⁴Cs EC decay. This case was chosen as the initial measurement due to its relatively large EC branching-ratio and short half-life, which are both well known and therefore suited to providing a benchmark test. The summed data over ~48

Multiple ion-bunch stacking

For RIBs from ISAC with yields $\geq 10^{6} s^{- 1}$ , the limiting factor for ion-storage in the EBIT is the space-charge of the RFQ (~10⁵–10⁶) [34]. For nuclei with small branching ratios ( $< 10^{- 4}$ ), this limit would exclude the possibility of performing statistically significant measurements within a reasonable amount of time. As a result, a method for overcoming this space-charge limit was recently tested using a beam of ^116g,mIn, with short (≈25 ms) RFQ accumulation times, and the subsequent injection of

Summary and conclusions

In summary, a significantly improved in-trap decay spectroscopy setup has been developed using the TITAN facility at TRIUMF. The apparatus consists of 7 low-energy planar Si(Li) detectors which surround the TITAN EBIT; an open-access charge-breeding ion trap with a magnetic field of up to 6 T. The current goal of this new facility is to provide a low-background environment for the observation of weak EC branching ratios of the intermediate nuclei for $β β$ decay. The ion-trap environment allows for

Acknowledgements

TRIUMF receives federal funding via a contribution agreement with the National Research Council of Canada (NRC). This work was partially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), and the Deutsche Forschungsgemeinschaft (DFG) under Grant FR 601/3-1. T.D.M. and A.T.G. acknowledge support from the NSERC PGS-M and CGS-D programs, respectively. K.G.L. would like to thank Stephan Ettenauer for many useful discussions on this work and its history with TITAN.

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¹: Present address: Stefan Meyer Institute for Subatomic Physics, 1090 Vienna, Austria.

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The TITAN in-trap decay spectroscopy facility at TRIUMF

Abstract

Introduction

Section snippets

The decay spectroscopy trap

Planar Si(Li) detectors

Environmental effects

On-line commissioning

Multiple ion-bunch stacking

Summary and conclusions

Acknowledgements

Nuclear Instruments and Methods in Physics Research Section B

Nuclear Instruments and Methods in Physics Research Section A

International Journal of Mass Spectrometry

Progress in Particle and Nuclear Physics

Physics Letters B

Nuclear Instruments and Methods in Physics Research Section B

Nuclear Instruments and Methods in Physics Research Section B

International Journal of Mass Spectrometry

Nuclear Instruments and Methods in Physics Research Section A

International Journal of Mass Spectrometry

Nuclear Instruments and Methods in Physics Research Section A

International Journal of Mass Spectrometry

Nuclear Instruments and Methods in Physics Research Section A

Nuclear Instruments and Methods in Physics Research Section A

Nuclear Instruments and Methods in Physics Research Section A

Nuclear Data Sheets

Physics Letters A

International Journal of Mass Spectrometry

Nuclear Instruments and Methods in Physics Research Section B

Radiation Detection and Measurement

Physica Scripta

Physical Review C

Physica Scripta

Physical Review Letters

Physical Review C

Physical Review Letters

Physical Review Letters

Physical Review Letters