Can beta radiation be detected in an expansion cloud chamber?

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Discussion Overview

The discussion centers on the detection of beta radiation in an expansion cloud chamber, comparing it to the detection of alpha radiation. Participants explore the reasons for the observed differences in track visibility between the two types of radiation, including energy levels, ionization density, and source activity.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that while alpha particles produce visible tracks in the cloud chamber, beta particles do not, leading to a question about the visibility of beta radiation.
  • Another participant explains that alpha particles are heavier and more ionizing than beta particles, which may affect their ability to ionize air molecules in the chamber.
  • It is suggested that beta particles may lack sufficient energy to ionize air molecules effectively, particularly at lower speeds.
  • A participant discusses the ionization density of alpha particles being high due to their charge and energy, while beta particles, being relativistic, have a lower ionization density and may not stop in the chamber.
  • There is a query about the activity levels of the radiation sources used, with specific values provided for strontium 90 and americium 241.
  • One participant references a source discussing stopping power and energy loss rates, suggesting that these factors may influence the visibility of ionization tracks.
  • Another participant points out the importance of controlling variables, such as the activity of the sources, when making generalizations about the observations.

Areas of Agreement / Disagreement

Participants express varying views on the reasons for the lack of visible tracks from beta radiation, with no consensus reached on the specific factors influencing this observation. The discussion remains unresolved regarding the visibility of beta radiation in the cloud chamber.

Contextual Notes

Participants mention the need for controlled variables, such as the activity levels of the radiation sources, to draw more general conclusions about the visibility of tracks in the cloud chamber.

CloudChamber
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Hello all,
I just built an expansion cloud chamber and noted that while my alpha source (americium 241) produced a plethora of tracks, the beta source (strontium 90) produced no tracks at all-just mist. Does anyone know if beta radiation can be seen in an expansion type cloud chamber? Why or why not? Details would be great!
Thanks
 
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I'm no expert but alpha particles consist two protons and two neutrons while a beta particle is simply a single electron (or positron). That is, an alpha particle is much heavier than a beta particle and so carries more energy at low speeds.
 
so you're saying that a beta particle wouldn't have enough energy to ionize the air molecules in a cloud chamber traveling at such a slow speed, whereas an alpha particle would?
 
The alpha particles are very heavily ionizing particles, all of which have the same initial energy. So all of the tracks are about the same length. The ionization density of the track in the cloud chamber is high because the alpha particles are slow, and the charge is high (2 protons). (Track ionization density is proportional to Z2)

The beta decay process involves the emission of both an electron and a (anti)neutrino. which share the energy of the decay of strontium 90 (and yttrium 90). So the energy of the betas ranges from 0 energy up to the maximum energy (about 2.28 MeV). Furthermore, because the betas are relativistic, the track ionization density is low. The energy of many of the betas is so high that they do not stop in the cloud chamber.
 
Let's step back a bit. What is the activity of each source?
 
Strontium 90 is around 140 Ci/g; americium 241 is 3.5 Ci/g.
 
I would think that 10 visible tracks per sec might be about right, so maybe 1000 dps into 4 π. 1000 dps = 0.03 μCi. If you are interested in researching this more, look at http://pdg.lbl.gov/2011/reviews/rpp2011-rev-passage-particles-matter.pdf, especially the plot of stopping power on page 4. The beta particles are near the minimum (about 2 MeV per gram per cm2), while the alpha particles are near the peak on the left (x4 because Z=2).
 
Yes, and how many grams of each do you have?
 
I'm not quite so sure... I took the americium 241 out of a smoke detector, so I'd assume there would be 2.0 times 10^-4 mg, the standard amount in scientific notation. As for the strontium, I have no idea. The website, however, says the activity level of the strontium 90 source is .1uCi.
 
  • #10
Wow, looks like a great source, Bob S. I haven't really started looking through it yet, but I took a quick glance and it seems that restricted energy loss rates actually result in less visible ionization. Any quick explanations for why this is?
 
  • #11
Typical smoke detectors are 1 or 2 uCi. That's 10-20x the activity of your beta source.
 
  • #12
Ah, I see. Without that variable controlled its hard to make generalizations.
 

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