Characterization of large area APDs for the EXO-200 detector

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Abstract

Enriched Xenon Observatory (EXO)-200 uses 468 large area avalanche photodiodes (LAAPDs) for detection of scintillation light in an ultra-low-background liquid xenon (LXe) detector. We describe initial measurements of dark noise, gain and response to xenon scintillation light of LAAPDs at temperatures from room temperature to 169 K—the temperature of liquid xenon. We also describe the individual characterization of more than 800 LAAPDs for selective installation in the EXO-200 detector.

Introduction

In recent years large area avalanche photodiodes (LAAPDs) have been discussed as photodetectors of scintillation light [1], [2], [3]. LAAPDs are compact, semiconductor devices with high quantum efficiency (QE) from the infrared to the vacuum ultraviolet (VUV). Made mostly of high purity silicon, very lightweight, and fabricated in a clean room environment, they can be produced with low intrinsic radioactive contamination [4]. Hence they are well suited to the detection of scintillation light in ultra-low background experiments, especially when the wavelength to be detected is in regions of the spectrum that are problematic for photomultiplier tubes (PMTs). This is the case for noble elements as the scintillation light is in the VUV region [5]. Although several groups working on the detection of low energy, rare events have been investigating LAAPDs [6], [7], Enriched Xenon Observatory (EXO)-200 is the first such detector to make wide use of them.

Section snippets

The use of LAAPDs in EXO-200

EXO is a program aimed at building a ton-class neutrinoless double beta (0νββ) decay [8] detector using xenon enriched to 80% in the isotope 136 as the source and detection medium [9]. While the EXO collaboration is planning to build a ton-scale detector with the ability to retrieve and identify the 136Ba atom produced in the double beta decay of 136Xe, an intermediate scale (200 kg of enriched xenon, 80% 136Xe) detector, without the barium tagging feature, EXO-200, is currently close to the

LAAPD handling and initial qualification

A total of 851 production LAAPDs were purchased for EXO-200 and produced by API between September 2006 and June 2008. Great care has been taken at all times in the handling of these LAAPDs, both to protect the unencapsulated diodes from damage and to maintain the ultra-low radioactive background and xenon purity requirements of EXO-200. In particular, unencapsulated diodes are adversely affected by humidity, so the devices are transported in a dry-nitrogen filled container and stored in a dry

Multiple-LAAPD test setup

The testing and characterization of the production LAAPDs is done in a vacuum chamber maintained at better than 10-6torr by a turbomolecular pump backed by a dry scroll pump. Fig. 3 shows a schematic of the test chamber and Fig. 4 displays a photograph of the system. Sixteen devices, mounted face down on a 9.5 mm thick copper disk, are tested simultaneously in the chamber. The copper disk connects together the anodes for all 16 LAAPDs and maintains them at a constant and equal temperature. The

Noise

Fig. 6 shows the measured dark noise as a function of temperature for fixed gains of 50, 100, 150, 200 and 250 for a typical LAAPD in the single-device setup. Electronic noise has been subtracted, and the RMS widths are given in electron equivalent charge. The noise performance of the device improves with decreasing temperature down to ~250K, with no observable change below that.

The intrinsic device noise includes contributions from both the capacitance and the leakage current. We have

Energy resolution

For accurate characterization of the LAAPDs gain and QE it is important to achieve good energy resolution in the 55Fe and XSLS spectra. Fig. 9 shows an example pulse height spectrum for the X-rays from the 55Fe source for a single LAAPD. The source produces X-rays of 5.90 (89%) and of 6.49 (11%). The two peaks are not fully resolved even at low temperatures where the device noise is low. The energy resolution is determined by fitting the main peak of this spectrum to a Gaussian distribution.

Gain

Figs. 13 and 14 show the gain of an LAAPD as a function of bias voltage and temperature. These data demonstrate the necessary requirements to maintain stable gain. At a gain of 100, corresponding to ~1400V bias, and at 169 K, the slope in the gain is approximately 1.5%V-1. The gain also increases rapidly with decreasing device temperature, approximately 5%K-1 near the operating temperature of 170 K. Hence to achieve a gain stability below 1%, the variation in voltage (temperature) must be below 1 

VUV response

We determine the relative QE for xenon scintillation light by measuring the response to XSLS events of each production LAAPD with respect to a reference LAAPD. Corrections are applied for the previously measured device gain and the location dependent XSLS brightness. The brightness at each of the 16 LAAPD locations is calibrated by comparing the response to the XSLS signal for a set of LAAPDs in multiple positions.

A histogram of the relative QE of the production LAAPDs is shown in Fig. 16. We

Stability

The stability of the multiple-LAAPD test setup is monitored by keeping the same device in the central position over many testing cycles. These central LAAPDs (a total of three have been used over the course of two years of testing), as well as several others that were characterized a second time several months after the initial characterization, also allow us to measure the stability of the devices themselves over time. Fig. 17 displays the measured gain of the central LAAPDs over time. No

Conclusions

We have performed a systematic set of measurements on 851 bare, low activity LAAPDs at room temperature and at about 170 K. We find that 565 devices comply with the EXO-200 requirements for use as VUV scintillation light detectors in the experiment (468 devices are needed). Most of the devices rejected have either high dark noise (~250 devices) or low relative QE (~30 devices).

Acknowledgments

We would like to thank the staff of Advanced Photonix for their cooperation in what turned out to be a rather exotic enterprise. We are also grateful to M. Szawlowski for his advice and guidance and to International Rectifiers for providing—on very short notice—epitaxial wafers at no cost. The Teflon used for the XSLS was provided free of charge by the DuPont corporation. EXO is supported by DoE and NSF in the United States, NSERC in Canada, FNS in Switzerland and the Ministry for Science and

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