1544 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 61, NO. 4, AUGUST 2014 Tissue Equivalence Study of a Novel Diamond-Based Microdosimeter for Galactic Cosmic Rays and Solar Particle Events Jeremy A. Davis, Susanna Guatelli, Marco Petasecca, Michael L. F. Lerch, Mark I. Reinhard, Marco Zaider, James Ziegler, and Anatoly B. Rosenfeld Abstract—Radiation detectors based on diamond are attractive for radiation protection applications in space and aviation due to their radiation hardness, large breakdown voltage, fast signal collection, and low noise, thus representing a valid alternative to silicon-based detector devices. In this paper, we study the tissue equivalence of diamond in proton and particle radiation fields typical of galactic cosmic rays and solar particle events. We deter- mine a method to convert microdosimetric response from diamond to water by means of Geant4 simulations. Results are presented showing that a simple geometrical scaling factor ( ) of linear dimensions is adequate to convert experimentally obtained microdosimetric energy deposition spectra in diamond to equiva- lent microdosimetric energy deposition spectra in water. Index Terms—Diamond, Geant4, microdosimetry, space radia- tion protection, tissue equivalence. I. INTRODUCTION N OWADAYS it is well known that the exposure to space radiation is a primary concern in manned interplanetary and low Earth orbit (LEO) missions as well as in aviation [1], [2]. Radiation effects can be deterministic or stochastic, poten- tially causing cancer or the death of crew members. In order to limit the risks associated with space missions and aviation, one strategy consists of monitoring the received dose. Microdosimetry is a methodology that allows for the mea- surement of the stochastic energy deposition distributions in mi- crometer-size sensitive volume (SV) within any arbitrary mixed radiation field [3], thus representing a valid radiation protection approach for space missions and aviation. Manuscript received September 12, 2013; revised December 06, 2013 and December 29, 2013; accepted December 30, 2013. Date of publication February 17, 2014; date of current version August 14, 2014. This work was supported in part by ARC DP DP1096600 . J. A. Davis, S. Guatelli, M. Petasecca, M. L. F. Lerch, and A. B. Rosenfeld are with the Centre for Medical Radiation Physics, University of Wollongong, Wol- longong, NSW 2500 Australia (e-mail: jad028@uow.edu.au; susanna@uow. edu.au; marcop@uow.edu.au; mlerch@uow.edu.au; anatoly@uow.edu.au). M. I. Reinhard is with the Australian Nuclear Science and Technology Or- ganisation, Lucas Heights, Sydney 2500, NSW Australia (e-mail: mark.rein- hard@ansto.gov.au). M. Zaider is with the Department of Medical Physics, Memorial Sloan-Ket- tering Cancer Center, New York, NY 10021 USA (e-mail: zaiderm@mskcc. org). J. Ziegler is with the United States Naval Academy, Annapolis, MD 21402 USA (e-mail: ziegler@SRIM.org). Digital Object Identifier 10.1109/TNS.2014.2298032 Diamond has been chosen as a candidate material to super- sede previous silicon-based microdosimeters developed at the Centre for Medical Radiation Physics (CMRP), University of Wollongong, New South Wales, Australia, with its collaborative partners [4], [5] because of its radiation hardness, large break- down voltage, fast signal collection, and low noise. A previous paper [6] has been published with the experimental characteri- zation of the first-generation novel device, showing the potential of creating electrically isolated 3D SV structures utilizing boron implantation within a pure diamond wafer. For radiation protection applications, the SV of the micro- dosimeter should be tissue equivalent. This means that the SV should respond similarly to tissue in terms of physical interac- tions when in contact with ionizing radiation. Diamond is often considered to be a tissue-equivalent material in a photon field by virtue of its atomic number ( ) being so close to that of tissue average ( ) [7]. However, the tissue equiva- lency of diamond in a charged particles field relevant to space radiation protection has not been yet investigated. In this work, we study a method to convert energy deposition spectra from diamond to water, and we compare the tissue equivalence of di- amond to silicon [4] for space radiation protection applications. This paper addresses the tissue equivalence of diamond with respect to protons and particles with energy spectra typical of the galactic cosmic rays (GCRs) and solar particle events (SPEs) components of space radiation as specified by the CREME96 model [8]. This work is done by means of Geant4 simulations [9], [10]. II. INTERPLANETARY SPACE RADIATION ENVIRONMENT From a radiation protection point of view, the most important components of the interplanetary space radiation environment include GCR and SPEs. GCR is composed of 87% protons, 12% helium ions, and 1% heavier ions. The energy range of GCR spans from eV/nucleon up to eV/nucleon [11]. GCR fluence is heavily affected by solar activity due to variations in solar wind pressure. GCRs are more efficiently scattered by the heliosphere during solar maximum than during solar minimum period, leading to an anticorrelation between solar activity and GCR fluence [12]. The composition of SPEs is also dominated by protons and particles with only a small contribution from heavy ions ( ). Solar particles are accelerated from solar flares or shock waves driven by coronal mass ejections (CMEs). The energy range of SPEs span from eV/nucleon up to 0018-9499 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.