Results on radiation tolerance of diamond detectors

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Abstract

In sight of the luminosity increase of the High Luminosity-LHC (HL-LHC), most experiments at the CERN Large Hadron Collider (LHC) are planning upgrades for their innermost layers in the next 5–10 years. These upgrades will require more radiation tolerant technologies than exist today. Usage of Chemical Vapor Deposition (CVD) diamond as detector material is one of the potentially interesting technologies for the upgrade. CVD diamond has been used extensively in the beam condition monitors of BaBar, Belle, CDF and all LHC experiments. Measurements of the radiation tolerance of the highest quality polycrystalline CVD material for a range of proton energies, pions and neutrons obtained with this material are presented. In addition, new results on the evolution of various semiconductor parameters as a function of the dose rate are described.

Introduction

Diamond has some unique characteristics suitable for detector applications in radiation harsh environments such as that of the future HL-LHC [1]. The large band-gap of 5.5 eV implies low intrinsic carrier density resulting in low leakage current and low noise but also roughly half of the signal collected with silicon devices. Other outstanding properties are the large displacement energy, which renders the lattice less prone to radiation damage, the low dielectric constant leading to lower capacitance for similar structure in other material, the high thermal conductivity and the high charge carrier mobilities, which allow fast signal collection. In addition, the low atomic number minimizes the particles scattering and absorption, which is desirable for detectors close to the interaction point. Hence, Chemical Vapor Deposition (CVD) diamond represents an interesting radiation tolerant technology, which may be exploited as sensor material that can be operated without cooling and the need of pn-junction. There are two types of CVD diamond material: single crystalline (scCVD) and polycrystalline (pCVD). scCVD is very pure, expensive and can only be grown on a single crystal seeds. pCVD diamond is cheaper and can be grown in large area wafers. However, it suffers from larger amount of intrinsic charge traps resulting in a smaller collected charge for a given thickness. For more than 20 years the RD42 Collaboration [2], [3] has developed CVD diamond detectors and studied their radiation tolerance. The latest results of the collaboration are presented in this article.

Section snippets

Device preparation and irradiations

Diamond samples, with a typical thickness of around 500μm, were cleaned with hot acid and etched with an oxygen plasma, then metallized with a single pad and tested with a source or in a beam test. Before being irradiated, they underwent “pumping” [4] with a 90Sr source to fill the intrinsic diamond traps, then the leakage current was checked to be below 10 nA. One scCVD diamond sample and three pCVD diamond samples were irradiated with 800 MeV protons at the Los Alamos Neutron Science Center

Results

For all measurements, the diamond samples were biased with two polarities at ±1 V/μm and ±2 V/μm [10], [11]. The amplified signal was calibrated for the conversion from ADC to electrons and corrected for pedestal by subtraction and corrected for common mode noise. The common mode corrected noise was around 80e for each sample. The precise particle position in the diamond samples, approximately 2μm, was determined with an high precision telescope made of 8 planes of silicon strip detectors plus

Conclusions

Radiation damage constants for CVD diamonds have been measured for different particle species at different energies. Those are compatible for pCVD and scCVD diamond samples indicating that both species follow the same radiation damage mechanism. The signal response figure of merit shows a linear dependence with proton fluence, increasing for scCVD and decreasing for pCVD samples, with indication of possible leveling after a certain fluence. A simple radiation damage model allows the MFP to be

Acknowledgments

The RD42 Collaboration gratefully acknowledges the staff at CERN for test beam time and their help in setting up beam conditions. We would especially like to thank Henric Wilkens, the test beam coordinator, for his assistance in making our tests a success. We would also like to thank the beam line staff at the Los Alamos Neutron Science Centre (LANSCE). We would especially like to thank Leo Bitteker of LANSCE for his assistance for carrying out the diamond detector irradiations in the blue room

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