Spectrometric performance of a HPGe semi-planar detector with large diameter

doi:10.1016/j.nima.2020.164712

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

Volume 985, 1 January 2021, 164712

https://doi.org/10.1016/j.nima.2020.164712 Get rights and content

Abstract

We present the results of the development of the technology for the growth of HPGe ingots with a diameter of 110–120 mm and the characteristics of the first detector made of a crystal of this diameter. A semi-planar detector with a crystal size of 110x35 mm provided an energy resolution of 570 eV (on 60 keV), 702 eV (on 122 keV), 1337 eV (on 662 keV) and 1830 eV (on 1332 keV) with noise level on pulser peak of 556 eV. The relative detection efficiency of gamma radiation of the detector on the energy of 1332 keV was 65%.

Introduction

Detectors based on high purity germanium (HPGe) crystals are the indisputable leaders among all types of gamma-ray detectors, providing excellent energy resolution with a sufficiently high detection efficiency [1], [2]. The high efficiency of gamma-ray detection is extremely important in almost all tasks related to precision radiation spectrometry, since it affects both the accuracy of the results obtained and the measurement time.

The design of HPGe detectors and their dimensions (diameter and height of the crystal) directly determine their detection efficiency in a specific energy range. The planar and semi-planar geometry of HPGe detectors, for which the crystal height is usually 2–3 times smaller than its diameter, provides a higher detection efficiency of gamma radiation compared with the coaxial geometry in the energy range below 1 MeV [3]. With the same dimensions of HPGe crystals, the planar and semi-planar detectors differ in the configuration of the electrodes. Due to the smaller size of the collecting electrode, the semi-planar configuration provides a lower electrical capacitance of the detector, and, therefore, a better energy resolution than the planar one. At the same time, the detection efficiency of a semi-planar detector in comparison with a planar one also turns out to be somewhat higher due to the fact that the field distribution in a semi-planar detector is closer to its distribution in a coaxial configuration [3].

This determined the success of using semi-planar detector in mining and in the processing industry when analyzing minerals and elements, the characteristic lines of which lie in the specified energy range, by nuclear-physical methods [4], [5], [6], [7]. At the same time, both the energy resolution and the detection efficiency of the detector in a specific energy range directly determine the achievable detection limits of elements in such tasks. Therefore, an increase in the detection efficiency of detectors, such as that which is due to an increase in their volume, is always relevant for such applications. However, the technology itself for growing crystals of detector quality, the rising of the grown HPGe ingots diameter, is a rather complicated technological task for all producers.

The company Umicore [8] currently grows and supplies on an industrial scale detector quality HPGe crystals with a diameter of up to 100 mm. These crystals typically have a concentration of deep levels (Cu) for p-type crystals below 4.5 $\cdot$ 10⁹ cm⁻³. The Hall mobility is $μ_{H}$ $\geq$ 10,000 cm²/V s and the carrier concentration range varies between (0.3–3.0) $\cdot$ 10¹⁰ at/cm³, depending on the application. The use of such crystals allows Baltic Scientific Instruments to manufacture coaxial detectors with a maximum detection efficiency of up to 160% and semi-planar detectors with a maximum diameter of 90 mm and a thickness of 35 mm [9].

In this paper, we present the results of the development of technology for the growth of HPGe crystals with a diameter of 110–120 mm and the characteristics of the first semi-planar detector made of a crystal of this diameter, confirming the high quality of the grown detector crystal. At the same time, the performance of the detector demonstrates the level of spectrometric characteristics that can be provided at the industrial level in the production of such detectors for analytical applications with gamma- and neutron activation analysis and other industrial applications.

Section snippets

HPGe ingot growth and performance characterization

For decades now, Umicore has been growing HPGe crystals with a variety of geometrical and electrical properties, depending on the customer needs. The crystals are grown in a hydrogen atmosphere using the Czochralski technique. As the presence of hydrogen gives rise to detrimental V2H (divacancy-hydrogen) complexes [10] inside the crystal, a controlled dislocation amount is intentionally introduced in the crystal to make those complexes inactive. However; it should be noted that too many

Detector manufacturing

A disk with dimensions D110 mm ×H35 mm had the mass of the crystal 1798 grams. A photo of the manufactured ready detector crystal is shown in Fig. 2.

The necessary geometry of the detector with a guard groove was formed by mechanical grinding with silicon carbide SiC. The damaged surface layer was removed by etching in a mixture of concentrated acids HNO3:HF (3:1). Next, a diode structure was formed having n+ and p+ electrodes. The n+ contact covers the outer surface of the crystal to the outer

Detector assembling

The fabricated HPGe crystal of the detector was mounted in an aluminum alloy holder, providing thermal contact, and fixed with clamping screws with fluoroplastic tips (Fig. 3).

The electrical contact to the n+ and p+ electrodes was provided by clamping gold-plated contacts with indium tips. The detector holder also has the input stage of the charge-sensitive preamplifier with resistive feedback, which is cooled together with the detector. The J309A field effect transistor and a 1 G $Ω$ feedback

$C$ – $V$ and $I$ – $V$ performance

To measure $C$ – $V$ and $I$ – $V$ performance, the detector crystal in the holder was installed in a deep stick cryostat cooled by liquid nitrogen. The measured $C$ – $V$ and $I$ – $V$ performance are shown in Fig. 4.

As can be seen from the $C$ – $V$ characteristics, studied in the range from 0 to 2000 V, the complete depletion of the manufactured detector occurs at a reverse voltage of 1200–1300 V. The capacitance of the detector at full depletion voltages was 7.7 pF.

$I$ – $V$ characteristics of the manufactured detector were

Conclusion

Thus, the developed technology for the growth of HPGe ingots with a diameter of 110–120 mm ensures a high quality of the grown crystals for the manufacture of large diameter gamma radiation detectors from them. A semi-planar detector made of a grown ingot with a crystal size of 110 × 35 mm had a gamma radiation detection efficiency of 65% for an energy of 1332 keV. The detector provided an energy resolution of 570 eV (on 60 keV), 702 eV (on 122 keV), 1337 eV (on 662 keV) and 1830 eV (on 1332

CRediT authorship contribution statement

R. Nurgalejev: Detectors crystals fabrication technology. S. Pohuliai: Spectrometric research studies, Article writing. A. Sokolov: Development of detectors crystals fabrication technology. V. Gostilo: Conceptualization, Methodology, Supervision. J. Vanpaemel: HPGe crystal growth technology development.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Funding sources

The results presented in this paper were obtained in the frames of research project No.14 “Research and development of an automated spectrometer for radioactive materials in which samples are exchanged by a robot”, supported by European Regional Development Fund (contract No.1.2.1.1/18/A/001 between the “Energy and Transport Competence Center” (http://etkc.lv/) and the “Central Finance and Contracting Agency of Latvia” (http://www.cfla.gov.lv/lv/).

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