Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Beta spectrometry problem, path of emmited particle

  1. Mar 21, 2012 #1
    Hi guys this is my first post since ive joined, so be nice!

    I've been playing around with a short lens spectrometer at uni, using a strontium-90 beta source. It basically consists of a long steel tube with the source housed at one end, and a solid state detector at the other, mid way there is a magnetic coil to generate the lens used to accelerate the beta paricles. There is an overall length from source to detector of 1.5m, and the tube has a radius of just over 10cm. placed dead centre of the tube is a mass of lead or a "lead-pig" as we call it, which absorbs gamma rays. the detector is connected to an amplifier, which is connected to a counter. I'm attempting to collect data to generate a kurie plot.

    When accelerated, the beta particle's trajectory is bent around the lead pig, and hits the detector,
    but when there is no magnetic field present, there is no acceleration of the beta particles.......

    In that case would I be right to assume that they would (generally) suffer a similar fate to the gamma rays, and be absorbed by the lead pig? in experiment, they do not appear to they still seem to curve around the pig, and hit the detector, and register as a count.

    the amount of counts at ~0 gauss is comparable to counts with the field on. I am quite puzzed as to how explain this occurence.

    now just to clarify, I ran a few tests with no source, and the system registered zero counts, so i dont believe there is any problems with the system. also the spectrometer makes use of three lead sheets, or baffles, that fillter out lower energy particles from the test. these do not seem to have any bearing on the problem.
    (however thay have been the root of some other unrelated problems :/)..

    does anyone have any idea how to explain this path of beta particles? i wouldnt imagine that the ssd id detecting scattered particles, but i could be wrong....

    What do you think? any similar findings or stories?
    Last edited: Mar 21, 2012
  2. jcsd
  3. Mar 21, 2012 #2
    Well, I can't say I understand what your machine is doing, but the one we have in our teaching lab doesn't do anything weird like that. When the field is off, the electrons that are travelling towards the detector just hit the pig, as you describe, and the ones that miss the pig just hit the inside walls of the chamber, since there is no field to deflect them back towards the detector. We then turn the field on and sweep it through a range of field strengths, and measure the counts you get at each step, and from this reconstruct the momentum distribution of the source electrons.

    When you say the counts at ~0 gauss are comparable to the counts with the field on, at what field strength is this? Does it matter? If you get no variation with field strength then something odd is going on, unless your spectrometer works somewhat differently to ours.
  4. Mar 21, 2012 #3
    This spectrometer is called a solenoidal beta-ray spectrometer. Using a lead plug in the center and lead baffles elsewhere is a common feature. Many articles were written between 1948 to 1960. The solenoidal magnetic lens does not accelerate (increase energy of) the betas, but instead deflects them in circular arcs in the transverse (azimuthal) plane. The focused betas go through 2Ï€ radians from source to detector, ending up on the axis of the spectrometer.
  5. Apr 1, 2012 #4
    Thanks for the replies, sorry for leaving it so long to get back to you.

    You're right, it is a solenoidal beta-ray spectrometer.

    Kurros, with the field at zero, i.e. no current flowing through the solenoid, you can get detect of the order of 700 counts over 180 seconds. At a Field strength of around 47mT you can acheive a similar value. as i mentioned before, I have tested the system without a source, and there were no counts (thankfully :P) now, I did determine, that the detector will count photons, (which is due to a flaw in the amplifier), so can the beta particles that deflect off the walls of the spectrometer, can they cause the emmision of photons? ( I doubt that would explain such a high count)

    another strange thing i noticed with these early high counts, the counts lower as the field strength goes up, untill about 35 to 39 mT where the rest of the count values seem to behave in accordance with expectations.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook