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Development of Detection Systems for Low-Energy Heavy Ions at DRAGON

Sabine Theis, C. Rolfs (Ruhr-Universität, Bochum, Germany); S. Bishop, A. Chen, C. Dale, J. D'Auria (Simon Fraser University, Burnaby, BC, Canada); U. Giesen (University of Notre Dame, South Bend, Indiana, USA); U. Greife (Colorado School of Mines, Boulder, Colorado, USA); R. Henderson, D. Hunter, D. Hutcheon, R. Openshaw, J. Rogers (TRIUMF); A. Shotter (University of Edinburgh, Edinburgh, Scotland)

Abstract

The new Dragon facility at TRIUMF is designed to measure alpha and proton capture reactions with radioactive ion beams in inverse kinematics. For nuclear synthesis the relevant energies lie in the 0.15 - 1 MeV/u range, where very low cross-sections are expected. Therefore detection of the recoil products among the beam particles will be a major task. This paper focusses on the end detectors, which will distinguish recoils from beam particles at the end of the DRAGON separator.

1 Astrophysical Background

Explosive nucleosynthesis, such as in novae, supernovae and x-ray bursts, plays an important role in the synthesis of elements heavier than oxygen. In these relatively hot environments, alpha and proton capture reactions of radioactive species might become significant. In particular most bottle-neck reactions that determine the breakout to higher-order burning processes need further study. One example of such a reaction is ²¹Na(p,g) which leads the burning in novae from the cold to the hot NeNa cycle. Better knowledge about their cross-sections could lead to more accurate models concerning stellar evolution and the production of the elements [1].

2 Introduction

At the new Detector of Recoils And Gammas Of Nuclear Reactions, DRAGON at the ISAC radioactive beams facility at TRIUMF, we plan to measure rates of key reactions in explosive nucleosynthesis down to energies of 0.15 MeV/u. Using inverse kinematics and a radioactive beam, we will study a range of reactions, starting with ²¹Na(p,g)²²Mg. With expected product-to-beam ratios as low as 10^-15, detection and separation of the recoils will be challenging [2].

3 The End Detector

Although the recoil mass separator rejects the beam coming from the windowless gas target with an estimated suppression factor of 10¹², some of these ions will reach the end detectors after charge changing or scattering collisions. Because of the inverse kinematics, they will have almost the same momentum as the recoils. Therefore we need a detector which is not only able to detect heavy recoils (A=13-24) of very low energies, but also to distinguish between the recoil products and ion beam particles [3]. This separation will be aided by a potential BGO gamma array, surrounding the gas target. Besides the energy detection of the emitted photons, the array provides also background suppression through coincidence measurements with the pulsed beam.

3.1 Time-of Flight

A "local" time-of-flight measurement over a distance of approximately 50 cm follows at the end of the DRAGON. Two options are being considered. The first approach utilizes a microchannel plate detector (MCP) and a parallel grid avalanche counter (PGAC) coupled internally with an ionization chamber (IC) for start and stop and energy signals. The second approach uses two MCP counters to determine the time, with an Si detector for total E detection.

Figure 1: Two different arrangements for the end detector

3.2 Microchannel Plates

To generate a fast timing signal, we detect secondary electrons produced by the particles hitting a carbon foil. Microchannel plates have proven to be an optimal device for fast electron amplification. While literature shows a wide range of fancy geometries to accelerate the electrons to the plates, the main feature is an equal path length for all electrons to achieve a proper timing signal. In addtion we would like to obtain information on the position of the incident particle to determine the angle. By comparing this angle with the emission of the gamma ray, we hope to further suppress the beam. Resolutions of 0.5 ns and 2 mm respectively are required. Therefore we decided to start tests with a so called electric mirror [4]. The ions cross a foil perpendicular to the beam. Secondary electrons are accelerated towards a parallel grid and finally reach the mirror where they are reflected at an angle of 90^o and accelerated towards the MCPs, preserving their spatial information. To avoid using a second foil or the PGAC for the stop signal, we might be able to collect secondary electrons produced in the entrance window of the IC or in the surface of the Si-detector. However this possibility has still to be tested.

Figure 2: Microchannel plates with the electric mirror

3.3 Parallel Grid Avalanche Counter

The PGAC is coupled to the IC. Incoming particles ionize the gas, while the ionization electrons create signals on the three grids of the PGAC, which lie in front of the active volume of the ionization chamber. The measurement of the charge deposited along the wires provides timing and a two dimensional position signal.

3.4 Ionization Chamber

The detection of heavy ions at energies below the Bragg peak pushes the perfomance limits of an ionization chamber. The aim is to achieve an intrinsic energy resolution of about 1% and to optimize the discrimination between beam and reaction products. Therefore the IC has a flexible design with 25 isolated anode strips over a length of 25 cm, that can be combined as desired depending on the specific reaction. We hope to separate the groups of interest further through E-DE discrimination.

Figure 3: Ionization Chamber

4 Summary

We are still continuing to test the end detector system, in order to determine the best configuration. According to the present schedule we hope to begin the experiments early in 2001.

References

[1]: C. Rolfs & W. Rodney, Cauldrons in the Cosmos, University of Chicago Press (1988).
[2]: The DRAGON at ISAC, funding application to NSERC (1997).
[3]: J. D'Auria, Proceedings of the RNB2000 Conference (2000).
[4]: P. Boccaccio et al, Nucl. Instr. & Meth. in Phys. Res. A243 (1973) 599-600.

http://www.triumf.ca/people/baartman/pap/RNB2000/Theis/Poster.html

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