CdZnTe for Simultaneous Detection. 241-Am source. X-rays/gammas. alphas. Am-Pu-Cm source. Voltage [mv] Voltage [mv]

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1 Development of CdZnTe for use as a tool in Nuclear Spectroscopy: GR/ M37325/1 Review Report October 1998 { October 2 1. BACKGROUND This fast track grant was aimed at the development of the semiconductor material Cadmium- Zinc-Telluride as a detector for use in nuclear spectroscopy, with the specic aim of investigating its potential for dual photon-charged particle detection. All of the main objectives of the proposal have been accomplished, with future work based on the results from this project being planned. This report summarises the work performed under the auspices of the EPSRC fast-track grant given to Dr. P.H. Regan to investigate the potential of the semiconductor material Cadmium-Zinc-Telluride (CdZnTe) for use as a channel selection detector in nuclear spectroscopy studies. This material has been identied as a viable semiconductor for such purposes due it relatively large bandgap (compared to germanium and silicon), which results in a reduced probability of random, thermal excitations. This feature allows reasonable photon energy resolutions at room temperture operation. The main objective of the grant was to investigate if such a detector could be used to measure charged particles simultaneously with low energy gamma-rays, in the same radiation environment. The work on this grant was undertaken predominantly at the University of Surrey, with help from two masters level students, A. Divoli (MSc in Radiation and Environmental Protection, Graduated September 1999) and K. Owen (MPhys Physics with Nuclear Astrophysics, scheduled to graduate June 21). Additional help was provided by Mr. C. Bishop, who was the named technician on the grant and by two EPSRC RAs, (Drs. C.J. Pearson and S.M. Vincent) who are funded on other grants associated with the development of novel radiation detector devices (see below). In addition to the locally based development work, three detectors were purchased and used for a new application at the Lawrence Berkeley National Laboratory (LBNL), Berkeley, California, namely to search for `superheavy' elements. This part of the project was carried out by the in-house group at LBNL, with help from an MPhys research student (N. Seward) from the University of Surrey. The LBNL group performed a variety of test experiments at Berkeley using these detectors over the period between January and November 2. The results of these experiments are still undergoing analysis. The outcomes of this research are intimately linked to Dr. Regan's other EPSRC research grants, which focus on the spectroscopy of nuclei with with exotic proton to neutron ratios and the developmentofnovel radiation detection devices to identify such decays. Following this fast track grant, Dr. Regan has had ve further applications for funding approved by the EPSRC, (GR/M91945/1, GR/M823/1, GR/M49625/1, GR/R17133/1 and GR/M5113/1). On four of these grants he is named as the principal investigator. 1

2 2. Key Advances and Supporting Methodology The project consisted of a number of parallel aspects, specically (i) the construction of the vaccuum test-rig; (ii) the design and construction of a data acquisition system to inspect and analyse the detector output signals; (iii) the purchasing and testing of the detectors using standard alpha-particle and gamma-ray sources; and (iv) in-beam tests of the detectors' charged particle response using the Surrey Van de Graa facility. As mentioned above, three of the detectors were also used in preliminary tests at the Lawrence Berkeley National Laboratory at the focal plane of the Berkeley Gas-lled Separator (BGS) for applications in the synthesis and identication of superheavy elements. 2.1 Construction of Vaccuum Chamber A simple roughing and turbo pump combination were purchased to provide a safe vaccuum environment for the charged particle source tests. A vaccuum chamber was constructed along the design specications of the YRASTBall target chamber at Yale University. This was done with the aim of investigating CdZnTe detector array geometries in a realistic target space. (Note that Dr. Regan has recently submitted a grant request to EPSRC under the Instrument Development Call to construct a full CdZnTe array based on the YRASTBall target chamber geometry). 2.2 Initial Bench Top Detector Tests A simple, PC based multi-channel analyser system was purchased together with the APTEK multi-channel analyser software package. This system also included a spectroscopy amplier card and therefore could be used to display test spectra if fed a simple output from the detector pre-amplier. In later tests, a full logical system was constructed, based on a sixteen channel CAEN dual gain amplier, which allowed two dierent gain outputs from the same pre-amplier input pulse. The latter system was purchased specifically with the dual gain requirements of a simultaneous high-gain photon and low-gain charged particle system in mind. The detectors, purchased from ev Products Limited, were 1 cm square by 2 mm thick spectroscopy grade CdZnTe crystals, mounted on PCB backings. The rst batch of detectors had a front contact which was made from a rather thick layer of gold leaf. The reponse of the rst detectors to alpha particles and low-energy gamma-rays was tested using both and sealed triple-alpha sources consisting of 239 Pu, 241 Am and 244 Cm. It was subsequently discovered that the attenuation of the alpha-particle energy in this gold layer resulted in an unacceptable degredation of energy resolution with regard to the charged particle response. For the detectors purchased later in the project, the gold leaf was replaced with a thin ( 1nm), diused layer of platinum. This was found to signicantly improve the measured energy resolution for alpha particles. The pre-ampler signals associated with the detection of the 6 kev photon following the alpha decay of 241 Am, together with the pre-amplier signal associated with the 5.5 MeV alpha-particle decay are shown in gure 1. 2

3 3 (a) 5 6 MeV α particles CdZnTe for Simultaneous Detection Am source X-rays/gammas Voltage [mv] Time [ns] Counts (b) 6 kev γ rays γ Energy [kev] Am-Pu-Cm source alphas Voltage [mv] Time [ns] Counts Time [µs] α Energy [MeV] Figure 1: Pre-amplier pulse shapes and nal spectra for alpha particles and 6 kev gamma-rays from the triple alpha source. The initial tests used a standard spectroscopy pre-amplier for the detector output signals. This was however replaced in the later tests with a small, hybrid ev 2167 amplier (see gure 2). The nal aspect of the preliminary `bench-top', test experiments used two CdZnTe detectors, operated in coincidence, placed around the open triple-alpha source. The aim of this test was to use one of the detectors in `photon mode' (ie. with a high amplier gain) and the other in `charged particle mode' (low gain). As gure 2 shows, the three main lines of the triple-alpha source were clearly observed in the raw, ungated, charged particle spectrum. However, when this same spectrum was gated by the coincidence requirement that a 6 kev photon was detected in the other CdZnTe detector, only the (ne structure) line associated with the 241 Am line remained. The results of these tests have been accepted for publication in Nuclear Instruments and Methods A and clearly highlight the eectiveness of such detectors for dual, detection. Note that the same pre-amplier was used for both detectors, only dierent amplication gains were used to select either photons or alpha particles. 3

4 3 25 Counts [1 1 ] FWHM 1 kev 5 2 Counts [1 ] α Energy [MeV] Figure 2: Left: Alpha-particle spectra from a triple ( 239 Pu, 241 Am and 244 Cm) alpha-source with and without the 6 kev coincidence to select the 241 Am ne structure line. Right: Detectors used (with thin Pt contacts), together with the hybrid pre-ampliers and a one penny coin for scale. 2.3 In-Beam Test for Response to Alpha Particles and Protons The nal part of the project tested the response of the detectors `in-beam' to protons and alpha particles of varying energies. This was achieved by using the nuclear reaction d( 3 He,)p, where the 3 He beam was provided by the Van de Graa accelerator at the EP- SRC funded University of Surrey Ion Beam Centre. The beam impinged on two deuterated polystyrene targets of thickness 4m and 45 nm respectively. The detectors were placed at backward angles in the vaccuum chamber, which is shown in gure 3. The large positive Q-value for this reaction means that at beam energies of the order of 1 MeV, the emitted protons and alpha particles are produced at energies of approximately 2 and 12 MeV respectively. By simple two body kinematics and taking into account transformations between the centre of mass and laboratory frames of reference, the energy of the emitted particles in the laboratory can be altered by varying either the primary 3 He beam energy and/or by moving the laboratory angle between the beam direction and the detector. Figure 3 shows the alpha particle and proton spectra obtained in the in-beam tests at dierent beam energies and detector angles. The quality of these spectra demonstrate the 4

5 Si Beam in CdZnTe θ Target position 5.7V Amplified pulse Preamplifier signal 1.7V Protons Protons 184mV 1.2V Alphas Alphas 4 µ s Rise Time (ns) Figure 3: Upper left: Photo of vaccuum chamber set-up used for in-beam test. Lower left: Pre-amplier and amplier outputs for proton and alpha particles signal. Right: Alpha particle and proton spectra obtained with CZT detectors following the d( 3 He,)p reaction at various beam energies and detector angles. eectiveness of the CdZnTe detectors for charged particle measurement at room temperature. The pulse shapes corresponding the proton and alpha-particle amplier and pre-amplier signals are also shown in gure 3. The diering ranges of the two types of particles results in pulse shape variations, due to the dierent hole and electron transport times to the contacts. It is anticipated that this phenomenon can be utilised in the form of a simple `zero-cross-over' circuit to discriminate between dierent types of charged particle radiation in the CdZnTe. The Surrey group is currently investigating this aspect of the detector performance. 3. Project Plan Review The project was performed essentially along the lines outlined in the project plan as described in the original application. A CASE student could not be found to work on the project, however, the student manpower was provided by two MPhys students and one MSc thesis student. The in-beam tests and the superheavy element aspect of the research 5

6 were not orginally anticipated in the orginal grant application and made pleasant `extras' to the nal review. 4 Research Impact, Dissemination and Benet to Society The work outlined above, highlighting the simultaneous alpha-particle/photon detection capabilties has been accepted for publication in Nuclear Intruments and Methods A. The work on the detection of protons and alpha-particles from the in-beam tests at Surrey has been submitted to the same journal and is awaiting referees' comments at the time of writing this report. In addition, the initial results were presented to a large, international nuclear physics audience in the form of an invited talk by Dr. Regan at The 2 nd Biennial Workshop on Nuclear Structure Physics Near the Coulomb Barrier: Into the 21 st Century, hosted at Yale University, USA, June Additional dissemination has come about in the form of a poster presented at the University of Salford Institute of Physics Congress. This corresponded to work carried out in collaboration with the group at the University of Liverpool, who performed some simulations of the response of the detectors which were compared with the data obtained from the bench-top source tests performed at Surrey. 5 Explanation of Expenditure The summary of the expenditure of this grant is attached to this report. The project was completed on budget with the only variances greater than 2% on the individual headings being an overspend of $ 1,821 on consumables, oset by small underspends on the travel and equipment lines. The main consumables overspend accounted for items such as higher costs for electrical cables and connectors than originally budgeted, together with extra funds required for the vaccuum equipment. Additional expenses under the consumables line included an upgrade in disc capacity and memory for the data acquisition and analysis PC. 6 Further Research The results from this grant have stimulated a signicant degree of interest in the international nuclear spectroscopy community. They also formed the basis for an application to the recent (November 2) EPSRC Instrument Development Call to build a full scale inner-ball array of CdZnTe for use as a `tagging' detector for spectroscopic studies of nuclei with exotic proton to neutron ratios. We are also using the equipment purchased as part of the fast track grant to investigate the use of pulse-shape discrimination to distinguish between dierent species of particulate radiation impinging on the detector. A prototype multi-element CdZnTe array, constructed from the detectors purchased in this grant is planned to be tested at the Yale University WNSL facility in mid-21. 6