1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Alpha particles with RBE, energy question

  1. Dec 10, 2006 #1
    1. The problem statement, all variables and given/known data

    Alpha particles with an RBE of 13 deliver a 53 mrad whole-body radiation dose to a 65 kg patient.
    (a) What dosage, in rem, does the patient receive?
    (b) How much energy is absorbed by the patient (in mJ)?

    2. Relevant equations

    dose in rem = dose in rad x RBE

    3. The attempt at a solution

    I correctly answered part (a), getting 0.689J/kg as my answer. For part (b), I then multipled the answer from (a) by 65kg, which equals 44.785 J, or 44,785 mJ. However, my answer is incorrect. Is there something I am missing? Thanks.
     
  2. jcsd
  3. Dec 11, 2006 #2

    OlderDan

    User Avatar
    Science Advisor
    Homework Helper

    My guess is you should be using rads for part b, since that is the measure of the actual energy, while rem is some attempt to quantify the damage caused by a certain amount of energy absorption.
     
  4. Dec 11, 2006 #3
    Part a) should still be in units of rem so that it is apparent that this is absorbed dose in a human. For part b), the actual energy that is absorbed has nothing to do with the RBE. RBE is only a measure of a tissue's sensitivity to a type of radiation, not an energy (well, except for neutrons which have varying RBEs depending on neutron energy).
     
  5. Dec 11, 2006 #4
    So what equation should I be using? My book only gives me:
    RBE = (dose of 200-kV X-rays necessary to produce a given biological effect)/(dose of a particular type of radiation necessary to produce the same biological effect)
    and dose in rem = dose in rad X RBE
     
  6. Dec 11, 2006 #5

    OlderDan

    User Avatar
    Science Advisor
    Homework Helper

    It should also give you the definition of a rad as 0.01 Joules per kilogram of mass.

    http://www.unc.edu/~rowlett/units/dictR.html
     
  7. Dec 11, 2006 #6
    Hmm, thanks. So (68.9 rad)(13) = 895.7 J/kg
    895.7 J/kg x 65 kg = 58220.5 J = 58220500 mJ?
     
    Last edited: Dec 11, 2006
  8. Dec 11, 2006 #7

    OlderDan

    User Avatar
    Science Advisor
    Homework Helper

    There is no 13. 53mrad = 53*.01 mJ/kg (the m in mJ comes from the m in mrad). Multiply that by the body mass and you get mJoules of energy. The RBE of 13 means that this much energy from alpha particles is more damaging than the reference radiation that corresponds to RBE = 1, but it does not change the actual energy absorption.
     
  9. Dec 11, 2006 #8
    So my answer should be 3445 mJ? It's still incorrect.
     
  10. Dec 11, 2006 #9

    OlderDan

    User Avatar
    Science Advisor
    Homework Helper

    That is not correct. You left out the factor of 0.01

    It is 53 mrad * .01 J/kg/rad * 65 kg
     
  11. Dec 11, 2006 #10
    Oh, so 34.45 mJ?
     
  12. Dec 12, 2006 #11
    Bump, can someone please check my answer?
     
  13. Dec 12, 2006 #12

    OlderDan

    User Avatar
    Science Advisor
    Homework Helper

    Do you really need someone to confirm that you multiplied correctly?

     
  14. Dec 12, 2006 #13
    Sorry, I was running out of attempts on WebAssign, and didn't want to risk it by misinterpreting your advice. Thank you!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Alpha particles with RBE, energy question
Loading...