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<a https://www.thestudentroom.co.uk/showthread.php?t=7467335'> Year 12s: what will be the first way you research your future options? >>
<a https://www.thestudentroom.co.uk/showthread.php?t=7467335'> Year 12s: what will be the first way you research your future options? >>

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Quality Factor and Ionising Radiation

Im studying the OCR physics B course and am having really trouble explaining how and when to use QF. its not in any of my text books and its not even on the friggin' million page, syllabus and even a google search didnt really clear things up for me.

Quality factor is also called radiation weighting factor which makes alot of sense and i appreciate that different particles have different QFs. Neutrons have a QF of 10 for example right? but i keep getting questions with QF and never quite know what to multiply/divide/whatever it with...

any help would be much appreciated
Reply 1
Avatar for Parkerd
Parkerd
OP
I suppose it would be useful to give an example question so:

Calculate the average rate at which a 65Kg person must absorb 0.025eV neutrons from the reactor over a year to reach the maximum permitted dose equivalent. (which is 20mSv from another question)

Neutrons have a quality factor of 10.
1 year = 3.2X10^7 s
1eV = 1.6X10^-19 J


Neutron Absorbtion Rate=.........................Bq
Reply 2
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Parkerd
OP
lol Cmon people! Just because this isnt about lesbian sex :P Absolutely no dis-respect to the thread starter for that, but its a fair comment that just about anything is more intersting than my physics questions!

and i realise im being really impatient here... meh revision does that to you : /

:biggrin:
Reply 3
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Parkerd
OP
please guys :'(
Reply 4
Avatar for 12358
12358
The expression i've been taught is Dose in sieverts = Dose in grays x Quality factor where 1 gray = 1 J per kg.

So in terms of this question, each neutron has 0.025eV of energy which = 0.025 x 1.6x10^-19 = 4x10^-21 Joules.
To get the dose in grays, divide by the mass i.e. 4x10^-21/65 = 6.15x10^-23 Gy.
In sieverts this = 6.15x10^-23 x the quality factor (=10) = 6.15x10^-22.
Since the question asks for a dose equivalent to 20mSv and each neutron is effectively a dose of 61.5x10^-23 Sv, 20x10^-3/61.5x10^-23 ie 3.25x10^19 neutrons are needed over the year to produce a dose of 20mSv.
Finally in units of Bq (per second) that is 3.25x10^19/3.2x10^7
= 1.02x10^12 Bq.

I just remember that a dose in units of grays is just energy (in J) per kg, irrespective of what causes the ionisation whereas in sieverts you take the source of ionisation into account by mulitplying the energy per kg by its quality factor. (Dose in sieverts = dose in grays (J/kg) x quality factor.

Sorry if that wasnt a particularly good explanation, someone else will probably give a better one at some point but hope that helps. :smile:
Reply 5
Avatar for Parkerd
Parkerd
OP
Perfect thanks a lot mate, that is really much appreciated :biggrin: i notice your going to be a UCL medic hopefully, a girl i know quite well is in the same position, best of luck getting what you need :wink:
Reply 6
Avatar for 12358
12358
No problem, glad i helped and thanks :smile:.
Good luck in the exams!

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