Ion beam induced charge characterisation of a silicon microdosimeter using a heavy ion microprobe Iwan Cornelius a, * , Rainer Siegele b , Anatoly B. Rosenfeld a , David D. Cohen b a Centre for Medical Radiation Physics, University of Wollongong, Northfields Avenue 2522, NSW, Australia b Australian Nuclear Science and Technology Organisation, PMB 1, Menai 2234, NSW, Australia Abstract An ion beam induced charge (IBIC) facility has been added to the existing capabilities of the ANSTO heavy ion microprobe and the results of the first measurements are presented. Silicon on insulator (SOI) diode arrays with mi- croscopic junction sizes have recently been proposed as microdosimeters for hadron therapy. A 20 MeV carbon beam was used to perform IBIC imaging of a 10 lm thick SOI device. Ó 2002 Published by Elsevier Science B.V. PACS: 87.53.Rd; 41.75.Ak Keywords: Microprobe; Microdosimetry; IBIC 1. Introduction Microdosimetry is used in the study of radio- biological properties of hadron therapy beams [1]. The radiobiological effectiveness of a therapeutic beam is dependent on the distribution of particle types and their energies. A useful characteristic of the beam is the spectrum of linear energy transfer (LET) values of all primary and secondary ions. This spectrum is conventionally measured with a gas filled tissue equivalent proportional counter (TEPC) which registers energy deposition events in a simulated microscopic volume. The TEPC does however have several disadvantages which are outlined in the literature [1]. The Centre for Medical Radiation Physics has been investigating an alternative technique using microscopic arrays of reverse biased pn junctions. A prototype SOI microdosimeter was developed and measurements were conducted at boron neutron capture therapy (BNCT) [2], proton therapy (PT) [3], and fast neutron therapy (FNT) [4] facilities. During these studies the potential for silicon microdosimetry in hadron therapy was realised. The SOI diode array is manufactured on a bonded SOI wafer with thickness 2, 5, or 10 lm. Several diode sizes are available on each device and all diodes in the array are connected in par- allel. The array size predominantly used in the research is 120  40 diodes with each pn junction having an area of 10  10 lm 2 . The total size of each diode cell is 30  30 lm 2 giving an array area of 0.044 cm 2 . The n þ and p þ silicon regions were constructed with Arsenic and Boron implantations at 30 keV at a fluence of 5  10 15 cm 2 . The im- purity concentration of the p type silicon is 1:5  10 15 cm 3 . Fig. 1 gives a cross-section of a single diode. Nuclear Instruments and Methods in Physics Research B 190 (2002) 335–338 www.elsevier.com/locate/nimb * Corresponding author. Fax: +61-2-4221-5944. E-mail address: ic02@uow.edu.au (I. Cornelius). 0168-583X/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII:S0168-583X(01)01316-7