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Current Measurement of Incident Proton ?

  1. Jul 24, 2014 #1
    The proton current in cyclotron are normally measured by Faraday Cup.
    However, I do not understand that if there is target material for the proton beam.
    Proton is fully stopped in the target material, for example, water.
    How the current is measured if it did not hit anything else?
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  3. Jul 24, 2014 #2

    Simon Bridge

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    The faraday cup current is, then, telling you the flux that did not interact with the target.
    If you want to know the flux that interacts with the target, then subtract the faraday current from the beam flux.
  4. Jul 24, 2014 #3
    The beam is fully stopped in the target material (light blue) before reaching x.
    I'm curious is this how the cyclotron that produce medical isotope measure beam current.
    Please advise me that if there is any reference about it.

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  5. Jul 24, 2014 #4

    Simon Bridge

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    The diagram shows "a simple model for conductive heat transfer" and does not appear to have anything to do with protons.

    One way to determine the beam flux is to do a control run without the target.
  6. Jul 25, 2014 #5
    I'm sorry not that the diagram is not clear.
    A typical radioisotope target system is like:
    The proton beam coming from the left, passing through foil, then hits the water.
    A control run without the target is a good way, but the flux during irradiation is not constant.
    Where should the current meter connect with the target to measure the current hitting the water?

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  7. Jul 27, 2014 #6

    Simon Bridge

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    You can sample the flux during irradiation with a small faraday cup or other sort of detector in the beam before the target. This does remove some of the flux from the beam, but you either arrange for that to be small compared with the total flux or you take the measurement into account - a bit like using a thermometer to measure temperature (it removes some heat from the system being measured).

    You can also get flux as a function of time during the control run ... this will help yu work out a strategy for dealing with the test run depending on what you hope to achieve. Projects like isotope manufacture only care about the mean flux since the test runs are long compared with beam fluctuations.

    Of course - you could put a detector between the target and the water - that's only there for safety.

    Note: the "typical system" in your diagram does not include a faraday cup - or any provision for detectors.
    It describes a commercial-type setup - which will already have it's own arrangement of detectors.
  8. Jul 27, 2014 #7
    Commercial-type setup like this one connect a current meter with the right (green) side of the target body.
    A colleague explains to me that once the proton enter the target, by the law of charge conservation, positive electricity carried by the proton must flows through the current meter if it's connected to the target body (metal, conductor).
    I'm just wondering can the positive electricity carried by the proton flows through water if the proton stops in the water ?
  9. Jul 27, 2014 #8

    Simon Bridge

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    Yeah - if the proton is completely absorbed in a solid target, and nothing else happens, then the target becomes slightly positively charged. This draws an electron from the wire that is attached to the target ...

    Presumably you could get water to do this. You'd certainly end up with more protons than electrons in the water ... and that charge has to go somewhere.

    This is nothing to do with a Faraday cup.
  10. Jul 27, 2014 #9


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    This is simple charge conservation - and the fact that even a tiny unbalanced charge leads to a huge electric potential. Your target will be close to neutral all the time, so if protons go in, there are some electrons flowing in as well. Current flow will use all available conductors, but if you provide the "best" connection to ground (lowest resistance) and attach a current measurement you should get a good estimate of the proton current.
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