EXPOSURE!?

Until it is thoroughly clarified, the term which by a linguistic definition describes “the state of being exposed to contact with something,” may befuddle uninitiated in the field of radiation protection.  While, indeed, although in an oblique way, it refers to “… exposed to contact … “more accurately, the term is a measure of something real and consequential- in the simplest: Density of the freed, or separated, electrical charges within a certain mass, that in the end, is a quantity of electricity in Coulombs per kilogram of mass.  But for the sake of accuracy and precision, let’s get formal: 

“Exposure X is the quotient of dQ by dm, where dQ is the absolute value of      

the total charge of the ions of one sign produced in air when all the electrons    

and positrons liberated or created by photons in mass dm of air are completely stopped in air 

X  =  dQ / dm 

The unit of exposure is coulomb per kilogram (C/kg). The special unit used

for exposure is the roentgen R, where 1 R = 2.58 x 10-4 C/kg. In the SI

system of units, roentgen is no longer used and the unit of exposure is simply 2.58×10-4 C/kg of air. 

JAN P. SEUNTJENS et al. in: Review of Radiation Oncology Physics: A Handbook for Teachers and Students CHAPTER 2. DOSIMETRIC PRINCIPLES, QUANTITIES AND UNITS. (Someone may “google” other, differently worded, definitions) 

Although we are trying to get rid of roentgen as a measure, it would stay with us for a long while and not only for historical reasons.  Wilhelm Conrad Roentgen was one of the greatest benefactors of mankind, for he selflessly granted the free use of his brilliant discovery to anyone who can learn how to manage it in a safe way.  Many did not and out of so many times a reckless use of the Roentgen devices and later radium and other radionuclides a radiation protection filed and profession was born.  Roentgen was the first practitioner of radiation protection for he never used his devices without wearing a lead lined apron.  Henry Becquerel within months followed Roentgen’s disclosure looking for X-rays in the mineral samples from around the Europe.  A piece of uranium ore wrapped in thick paper, inadvertently darkened his photographic plates and the radioactivity was observed.  Thus it can be said that the Roentgen’s great discovery, also, prompted a great discovery of radioactivity.

Understanding the basic principles of dosimetry is to comprehend the ionization processes and rates of ionization in different materials.  A quantity of the free and separated charges is a measure of the interactions of photons of different energies, of either: discrete or continuous spectra, and not only of how much energy is deposited into the considered state and amount of matter.  It is a complex set of circumstances that deserve some attention of the curious to whom this series of presentations is dedicated. 

Most of our measurements in the past, and today even more, are done by the measurements of some quantities of light and electricity and so it has been done at the times when the unit roentgen was defined. 

Under steady state, that is, equilibrium, and other, controlled conditions, in a small, central part of an air ion chamber, slides ## 5, 6, & 8, it is possible to, with a small voltage, extract a saturation current very closely resembling the amount of electricity corresponding to the total of the freed charges.  In the times of Wilhelm R, electric charges were measured in the cgs system’s electrostatic units, or esu of ~ 3.34 × 10-10 Coulombs slide # 7. Thus when a stream of collimated X-rays emanated from a Roentgen tube generated 1 esu/cm3 of air at STP, then that quantity of free ionic charges liberated per second, was defined 1 roentgen.  You can quickly re-compute this value to 2.58 x 10-4 C/kg of air, beginning with the notion that 1 m3 of the STP air, weighs ~1.293 kg. The rate of 1 esu/cm3s of air at STP, also, spells as 1 esu/1.293 × 10-3 g s of air. 

That the electric current was of granular nature, consisting of myriads of electrons of ~ 4.803×10-10 esu each, was also known at the time of the roentgen R, definition.  It was not so difficult, then, to compute the number of free charges of one polarity that will measure 1 roentgen in 1 esu or ~2.08 ×109

Converting the numbers from the cgs to SI units is, to some extent, cumbersome so I will not burden the reader with that here.  One fact ought to be, however, remembered: It is far more efficient to compute in terms of joules and MeVs then in other energy units; 1 J ~ 6.24 ×1012 MeV. Gray 1Gy = 1 J/kg and 1rad = 1Gy/100, are easily convertible to joule and roentgen when 1 J/kg µ ~114 R, 1 R µ ~5.47×1010 MeV/kg

The central point of the material .pdf Exposure & Exposure Rate, is, however, to point out the usefulness of the Specific Exposure Constant GX in Rm2/Cihr.  The Constant embodies the concept of Exposure rate due to photon emanation from 1 Ci at 1m distance, through an area i.e., 1 m2, that defines a strictly outlined space, that is, 1 steradian. Therefore: 

GX (Rm2/Cihr) is the radionuclide specific exposure rate unit

              

For the exposure rate due to a “point” source, we are accustomed to learn of the 6CEN and 0.5CEN, which without C or activity in Ci, is the Constant itself.  In my opinion, for our techs it would be far more useful to master the concept of the Specific Exposure Constant GX for we never, in practice, use the 6CEN and 0.5CEN.  I made a few slides to show the principles of using the 0.5 EN and 6 EN to compute GX  from raw decay data.  The GX has been, however, computed accurately for a great number of isotopes and their photon energies by far more competent people than I.  A few values are tabulated on slide # 24. 

The formulas for Exposure rates at 1 m and 1 ft are terribly simple: 

                    X*R/hr @1m = ( GX × C ) / d2      X*R/hr @ 1ft = ( 11GX × C ) / d2 

The C in the formulas is activity in curries Ci, while distances, that always ought to be involved in the computation, need to be in their respective units.  In fact, if we always keep in mind that 1ft closely approximates 0.305 meters (0.3048 m exactly), only the formula for Exposure Rate X* at 1 meter is necessary.  The factor 11 in @ 1 ft formula is used to bring closer computations using 0.5 CEN and 6 CEN formulas. 

For the curious minds, in the .pdf presentation, on one of the slides, I left a couple of computational tasks unanswered. I am challenging our techs – no supervisors, no NRRPT, or instructors; only plain techs, individuals or in a group, and even curious decons may participate in the race to provide the right answers along with the numerical work and explanation how the correct results are obtained.  The first to report with a post will receive RAD DECK playing cards a few of them shown below.  

                                                    

 

There may be some inadvertent mistakes in what I wrote here and in the .pdf  PPT presentations.  If you spot them, please let me know so I can correct them.  Thank you for reading and I hope you will also go to see the.pdf file.  Please leave a comment on what you find of interest there.  Your opinion is very valuable to all of us that either teach or practice radiation protection.

 What is Dose?

 What is Exposure?

Exposure.pdf