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Relativistic effects on radiation sources

  1. Jan 20, 2015 #1
    I have limited physics knowledge, but I have always been interested in the physics and I am an industrial radiation safety officer with an engineering background.

    Have there been any experiments in respect to measuring half life with large variance in gravitational fields?

    Although half life may be constant for a particular radioisotope, can I also assume this is only relative to a radioactive particle at rest?

    If a radioisotope was travelling away from me at 99.9% of the speed of light, would it not appear by measurement of alpha particles hitting a detector appear to have a different half life ?

    Where i also get confused is in respect to the relative gamma field.

    if the radioisotope was moving away, can I assume the gamma emission would remain the same, but that there would be a shift in the wavelength ?

    That is to say the gamma emission may end up being observed as visible light if the speed of the radioisotope was moving away fast enough?

    If this were true in respect to wavelength shift, how fast would a radioisotope like Cs137 need to be moving in order for its gamma emission to shift far enough down that it no longer poses a threat (no longer ionizing radiation)?
  2. jcsd
  3. Jan 21, 2015 #2


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    Staff: Mentor

    Yes. The reported half-life (and atomic mass, and just about all the other properties) are those that would be measured by an observer at rest relative to the material. From this, we can use the methods of relativity to calculate what an observer moving relative to the material would measure.

    Yes, and this effect is observed (with subatomic particles, not necessarily nuclei) every day in particle accelerators. Also, google for "cosmic ray muon time dilation" for another example.

    Yes, the Doppler effect will red-shift the gamma radiation from a source moving away from you. Google for "relativistic Doppler" to find the formula that you would use to calculate the speed needed to shift gamma radiation down into the visible spectrum. (It works the other way too - approaching a source of visible light at hyper-relativistic velocities would create gamma exposure).
  4. Jan 21, 2015 #3
    Thanks, I figured I had it right, but thought I might as well ask a few experts.

    Would I also be correct to say that you could shift the energy of a gamma emission by accelerating the radioisotope?

    example Cs-137 particle in a linear accelerator.

    If the emission is 661 KeV and your accelerator is 200KeV, would you observe an 861KeV gamma emission in one direction and a 461KeV gamma in the exact opposite?

    If I am using a detector to analyze and identify an unknown radioisotope, are there shifts in KeV due to movements of the atoms relative to the detector?

    that is to say I am wondering if the temperature of the radioisotope makes a difference when measuring the KeV spikes?

    I assume statistically it would work out to be what is published when observed over time, but can I assume that an instantaneous measurement would have some variance in the observed KeV levels due to movement of the atoms within the sample?

    Detectors interact differently in respect to measurement speed, so I am curious about the effects.

    I am unfamiliar with the calculation of speed at which atoms typically move within a sample material at a given temperature, I assume there is some type of formula.

    Would said formula have to take material Z into account?
  5. Jan 27, 2015 #4
    Cs-137 nucleus accelerated to 200KeV energy is moving relatively slowly (not relativistic), thus blue/red shifting is small. Cs-137 needs to have many GeVs of kinetic energy to have relativistic velocity.

    The temperature of 11600K is equivalent to average energy of thermal motions of only 1 eV.
    Unless you are measuring gammas from an early thermonuclear fireball, you can safely disregard Doppler shift due to thermal motions.
  6. Jan 28, 2015 #5


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    Well, even for slow motions, you can have some Doppler shift. It all depends on your detection mechanism. For example, spectroscopical observations in a lab (due to lasers) are "sensitive" even to small velocities of the atoms in your target. That's the reason some mechanisms exist to cool down the matterial + trap it.
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