3/2003 Rev 1 II.2.9a – slide 1 of 25 IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources Part IIQuantities and Measurements Module 2Dosimetric Calculations and Measurements Session 9aMicrodosimetry Session II.2.9a

3/2003 Rev 1 II.2.9a – slide 2 of 25 Overview  Microdosimetry theory and principles will be discussed  Students will learn about microdosimetry theory and principles, track descriptive approach, limitations on the concept of linear energy transfer (LET), site relevant approach, and various stochastic quantities created to support microdosimetry

3/2003 Rev 1 II.2.9a – slide 3 of 25 Content  Theory of microdosimetry  Track descriptive approach  Limitations on concept of LET  Site relevant approach  Stochastic quantities (energy deposit, energy imparted, specific energy imparted, and lineal energy)

3/2003 Rev 1 II.2.9a – slide 4 of 25 Theory of Microdosimetry  Microdosimetry most generally means determination of an absorbed dose on a microscopic scale of spatial distribution  More specifically, it is the science that deals with the spatial, temporal, and energy- spectral distributions of energy imparted in cellular and subcellular biological structures

3/2003 Rev 1 II.2.9a – slide 5 of 25 Theory of Microdosimetry  It includes the relationship of such distributions to biological effects  Microdosimetry seeks to express the "quality" of radiation in terms of subtle physical parameters sufficient to allow quantitative prediction of biological effects for different types of ionizing radiation

3/2003 Rev 1 II.2.9a – slide 6 of 25 Track-Descriptive Approach  Earliest approaches to microdosimetry focused on the rate of energy loss of the charged particles that deliver dose  Linear energy transfer (L  ) was defined and applied to radiobiological target theories  L  is a complex descriptive parameter

3/2003 Rev 1 II.2.9a – slide 7 of 25 Limitations on L   Range effect:  L  says nothing about the range of the particle or whether it can traverse a given biological target volume  If the particle stops in the target volume, L  is obviously irrelevant

3/2003 Rev 1 II.2.9a – slide 8 of 25 Limitations on L   Delta-ray production:  L  does not describe the diameter of a charged-particle track, only the rate of energy loss along the track  For example, a 10-MeV proton and a 167-MeV alpha particle have the same L  but the velocity (  = v/c) of the alpha particle is twice that (0.145) of the proton

3/2003 Rev 1 II.2.9a – slide 9 of 25 Limitations on L   Thus, the maximum energy of the  -rays generated by the alpha particle is 92 keV, while that for the proton is 21 keV  In water these  -rays would have ranges of about 120 and 9  m, respectively  The diameter of the alpha-track would be 13 times as large as that of the proton track in this case, and the average energy density per unit volume in the alpha track would be less than 1% of that in the proton track

3/2003 Rev 1 II.2.9a – slide 10 of 25 Limitations on L   Random variations (energy-loss straggling):  L  describes the expectation value of the rate of energy loss by a charged particle of a given type and energy  It does not address the random nature of energy losses along a track, which may leave zero energy in a small target volume or give it orders of magnitude more energy than would be predicted by L 

3/2003 Rev 1 II.2.9a – slide 11 of 25 Site-Relevant Approach to Microdosimetry  Different approach was needed to take into account the previously discussed limitations on L   The approach must account for the energy spent by radiation in a defined site volume  Approach was developed by Rossi (1955) and defined new stochastic quantities in terms of which the energy dissipated in microscopic sites by individual ionizing events could be stated

3/2003 Rev 1 II.2.9a – slide 12 of 25 Harald Rossi, Father of Microdosimetry

3/2003 Rev 1 II.2.9a – slide 13 of 25 Stochastic Quantities  Energy Deposited,  i  Energy Imparted,   Specific Energy Imparted, z  Lineal Energy, y

3/2003 Rev 1 II.2.9a – slide 14 of 25 Energy Deposit,  i This is the energy deposited in a single interaction, i:  i = T in – T out + Q  m

3/2003 Rev 1 II.2.9a – slide 15 of 25 Energy Imparted,  This quantity is defined as:  =   i  is to be expressed in joules or eV i

3/2003 Rev 1 II.2.9a – slide 16 of 25 Specific Energy Imparted, z This is the quotient of  by m, where  is the energy imparted by ionizing radiation to matter of mass m: z =  /m z is to be expressed in J/kg or grays

3/2003 Rev 1 II.2.9a – slide 17 of 25 Lineal Energy, y This is the quotient of  by l, where  is the energy imparted to the matter in a volume by a single energy-deposition event, and l is the mean chord length in that volume: Y = _ _l

3/2003 Rev 1 II.2.9a – slide 18 of 25 Boron Neutron Capture Therapy (BNCT) Microdosimetry

3/2003 Rev 1 II.2.9a – slide 19 of 25 Prototype Silicon-Based Microdosimeter

3/2003 Rev 1 II.2.9a – slide 20 of 25 Simulation of Alpha Particle Interacting in Silicon Microdosimeter

3/2003 Rev 1 II.2.9a – slide 21 of 25 Summary  Microdosimetry theory and principles were discussed  Students learned about microdosimetry theory and principles, track descriptive approach, limitations on the concept of linear energy transfer (LET), site relevant approach, and various stochastic quantities created to support microdosimetry

3/2003 Rev 1 II.2.9a – slide 22 of 25  Knoll, G.T., Radiation Detection and Measurement, 3 rd Edition, Wiley, New York (2000)  Attix, F.H., Introduction to Radiological Physics and Radiation Dosimetry, Wiley, New York (1986)  International Atomic Energy Agency, Determination of Absorbed Dose in Photon and Electron Beams, 2 nd Edition, Technical Reports Series No. 277, IAEA, Vienna (1997) Where to Get More Information

3/2003 Rev 1 II.2.9a – slide 23 of 25  International Commission on Radiation Units and Measurements, Quantities and Units in Radiation Protection Dosimetry, Report No. 51, ICRU, Bethesda (1993)  International Commission on Radiation Units and Measurements, Fundamental Quantities and Units for Ionizing Radiation, Report No. 60, ICRU, Bethesda (1998) Where to Get More Information

3/2003 Rev 1 II.2.9a – slide 24 of 25  Hine, G. J. and Brownell, G. L., (Ed. ), Radiation Dosimetry, Academic Press (New York, 1956)  Bevelacqua, Joseph J., Contemporary Health Physics, John Wiley & Sons, Inc. (New York, 1995)  International Commission on Radiological Protection, Data for Protection Against Ionizing Radiation from External Sources: Supplement to ICRP Publication 15. A Report of ICRP Committee 3, ICRP Publication 21, Pergamon Press (Oxford, 1973) Where to Get More Information

3/2003 Rev 1 II.2.9a – slide 25 of 25 Where to Get More Information  Cember, H., Introduction to Health Physics, 3 rd Edition, McGraw-Hill, New York (2000)  Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Table of Isotopes (8 th Edition, 1999 update), Wiley, New York (1999)  International Atomic Energy Agency, The Safe Use of Radiation Sources, Training Course Series No. 6, IAEA, Vienna (1995)