Dosimetry and Measuring Radiation

since radiation has many important applications for example radiation therapy that is used in medicine it is very important to be able to measure and quantify how much radiation is produced by some radioactive source or how much radiation is absorbed by some type of substance or material now the subject that deals with measuring and quantifying radiation is known as dosimetry and that's exactly what we're going to concern ourselves in this lecture so we're going to discuss four important terms that we must know we're never conducting though symmetry we're never dealing with those symmetry and let's begin by defining the source activity or the activity of our source now what exactly is a source well the source is basically some type of radioactive isotope or nuclei that readily undergoes radioactive decay and in a process releasing radiation now the source activity or simply the activity is the number of nuclei that decay every single second and there are two units that we must be aware of when dealing with the source activity unit number one is known as the Curie and unit number two is known as the Becca realm so one Curie is equal to three point seven zero times ten to the ten decays every single second and this is a relatively old unit the modern-day unit is the Becquerel and one Becquerel is equal to simply one decay every single second now what exactly does the magnitude of the activity actually depend on well we actually discuss this part when we spoke about half-life and radioactivity so the magnitude of the activity of our radioactive source depends on two things firstly it depends on the total number of radioactive nuclei that have not yet undergone radioactive decay at that moment in time and it's designated by uppercase n now the half-life of our radioactive isotope also determines the magnitude and the equation that gives us the source activity given by uppercase R is given by this formula so 0.693 divided by the half-life multiplied by uppercase n so let's move on to the next important term so let's discuss something known as the absorbed dose also known as the exposure so let's suppose we have some type of source some type of radioactive isotope that readily undergoes decay so as it decays it basically releases radiation into the surrounding atmosphere into the surrounding area and as that radiation is released that radiation interacts with the substance found in the surrounding area and so that material or substance can absorb some of that radiation and the absorbed dose basically describes how much radiation is absorbed by that surrounding material now the old unit that describes the absorbed dose is known as the roentgen the roentgen basically describes how much energy in joules is deposited by that radiation is transferred by that radiation to the surrounding air and one roentgen is defined as 0.87 a times since the negative two joules of energy that is transferred into one kilogram of the surrounding air so once again it's the amount of radiation is transferred specifically by x-rays or gamma rays now because we have many different types of radiation other than x-rays and gamma rays and because not only air but other types of substances and materials can also absorb radiation we no longer use the Rankin we use something called the rat so the modern unit of absorbed dose is known as the rad so basically the rat which is given by uppercase R describes how much energy is deposited by the radiation into some type of material per kilogram of that material and one rad is equal to one point zero zero times ten to the negative two joules of energy that is absorbed by one kilogram of that substance now our red is not the SI unit of absorbed dose the SI unit of absorbed dose or exposure is the grey and one gray is equal to one Joule of energy that is absorbed by one kilogram of surrounding material of our object that is absorbing by that radiation and this is equal to one hundred rats now notice the rat is given by uppercase R but this is not the same thing as this uppercase R this uppercase R has units of the Becquerel and it designates the source activity or the activity of our source this uppercase R designates the absorbed dose it's how many joules of energy is absorbed by one kilogram of our substance so once again the gray and the rat tell us how much energy is absorbed by one kill of any type of material for example this whiteboard our mark or the skin the bones in our body and so on now the question is with a question that I want to pose is the following what exactly determines how much radiation is absorbed by some object so not only does the type of radiation determine how much energy is absorbed by given object but also the type of material that we are using also the determines how much radiation is absorbed for example if we take x-rays and direct x-rays at our body most of that radiation will be absorbed by the bones of the body and not by the skin and the cells between the skin and the bone so not only there's a type of radiation that are determined how much of that energy is absorbed but also the type of material we are using determines how much energy is absorbed what our absorbed dose is so the question is what exactly determines the actual damage that is caused by radiation remember when radiation passes through matter it damages that matter it breaks the bonds and damages that structure as a whole and something called the quality factor is used to describe how much damage is actually caused so basically the gray and the red do not really tell us how much damage is caused to our absorb material it it only tells us how much of that energy is simply absorb now the quality factor of any radiation is the number of rads of x-ray or gamma radiation that produces the same image as one rat of that given substance so we arbitrarily choose the quality factor of x-rays or gamma rays to equal one so basically what this means is the following so since we're using x-rays or gamma rays that means one right of gamma rays or x-rays needs to be used to produce the same damage as one right of gamma or x-rays so that makes sense now however if we examine the QF the quality factor of a different type of radiation for example alpha particles we see that the QF is equal to 20 and that means we basically need 20 rounds of x-ray radiation to produce the same damage that is caused by one rad of alpha radiation so alpha radiation causes much more damage than x-rays or gamma radiation and finally let's define something called the effective dose the fact of those is simply the product of the number of rads of radiation we are using and the QF the quality factor of that radiation and the units of this is known as REM r.e.m

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Keywords: dosimetry, measuring radiation, radiation, source activity, curie, becquerel, Roentgen, rad, gray, quality factor, effective dose, absorbed dose, radiation exposure, relative biological effectiveness, nuclear physics

Id: 08eAe_C1mZ4

Length: 10min 13sec (613 seconds)

Published: Fri Apr 25 2014