Urgent public health warning issued over lost radioactive capsule

For simplicity lets assume 50 Liter is a 1 meter x 1 meter x 5 centimeter thick panel. 5 centimeter is usually sufficient to give you a good change to measure gamma rays at most energies and you want to have a large surface area exposed to the source, so this seems a quite optimal configuration for 50 Liter of NaI.

A source emits gamma rays in all directions uniformly, so to calculate the fraction of gamma rays that end up in our detector we calculate in "solid angle space".

The surface area of such the detector is 1 meter^2. The solid angle coverage can be approximated by surface area / distance^2 [1], so at 40 meter that would be about 0.000625. The full solid sphere is 4PI ~ 12.5, which is the surface area of a ball with 1 meter radius.

The fraction of the full solid angle that is covered by the detector is 0.000625 / 12.5 = 0.00005.

You have 19 Becequerel = 19 Gamma rays per seconds leaving the source so 0.00005 * 19 Bequerel * 60 seconds = 0.057 gamma rays per minute reaching the detector. You won't see this over the background.

Putting 37 Million Becquerel into this equation, we will get roughly 111,000 counts / minute in the detector. Remembering correctly (and this is only a rough guess) you would expect about 700,000 gamma rays per minute from background (what we call naturally occurring radioactive materials or NORM) in that much detector volume. So that is about a 1:7 signal to background ratio. You should have a very good chance of seeing that.

I neglected here that not all gamma rays that reach the detector actually will leave a signal in the detector. But at 662 keV, the energy most of the gamma rays in Cs137 are emitted at, about half of them do so in a 3 cm thick detector. At 5 cm I would estimate about 75% or more doing so.

Hope that gives you an idea how you can estimate these types of questions.

[1] https://en.wikipedia.org/wiki/Solid_angle

Edit: Forgot to mention the radon. Usually you do not measure much radon unless it rains. It is mostly a dessert there so I don't think that should be an issue

Isn't it just?

I'm glad you picked that up as 1.9 MBq through up to 750 MBq is more of the range I would have expected for a mining instrumentation source [1].

I'm guessing both the The Guardian [2] and AAP [3] relied on the same reporter who missed fact checking and an M (and possibly a decimal place) when interviewing the W.Australian Health minister (who, of course, may have bumbled that himself).

The W.Australian official press release avoided details [4].

Take note when going forward in your career as there'll likely be some moment of unexpected public relations in the future.

Any thoughts on the "full quote" ?

    Chief Health Officer Andrew Robertson said the small silver cylinder was a 19-becquerel caesium 137 ceramic source commonly used in radiation gauges.

    “That may not mean a lot to people but probably more concerning is that it does emit a reasonable amount of radiation,” he said.

    Dr Robertson said the unit emits about two millisieverts of radiation per hour, which is the equivalent of having 10 x-rays in an hour.

    “Two millisieverts is also the amount of natural radiation we would receive in a year just by walking around,” he said.

    “This is a source we have to be very careful of … It is quite a large radiation dose.”

I confess to a distaste for Sieverts as a per kilo absorbed dose (or is that Grays) as its kind of subjective to target rather than source properties (as the source may be partially shielded by lead, water, etc.)

Moving on, 700 k counts / minute background (or 12 k counts/sec for those of us working in one second over lapped accumulation windows) sounds ballpark.

The interesting twist on the source, though, is it might be shaped in the sense of having a wrapper of lead (say) with an aperture of preference to direct the radiation preferentially.

> I neglected here that not all gamma rays that reach the detector actually will leave a signal in the detector.

For at least two reasons, 'eh?

* Not all gammas passing through the crystal will cause a flash (you covered by implication), AND

* Not all flashes will be counted (oft neglected) as the scintillation counters get saturated by single events and need to recover, they also fluff damn near simultaneous multiple events as singles (all this dependant on scintillation detection equipment).

> Hope that gives you an idea how you can estimate these types of questions.

I was very interested to hear your reasoning, I appreciate it and I hope your comment gets a few eyeballs, although HN moving on apace as it does, who knows?

> Forgot to mention the radon. Usually you do not measure much radon unless it rains. It is mostly a dessert there so I don't think that should be an issue

Hmm. You might want to look into this further I suspect.

My poor understanding is that rainstorm fronts drop the barometric pressure which draws radon gas from the cracks and crevasses in which the gas has already been expressed from the rock sources.

So, yes, there is an increase in radon gas where radon gas is normally expressed due to pressure drops.

However, in a hypothetical environment with no pressure changes .. radon continues to be expressed if it is being created.

I've found, in my humble experience, that even on clear days rain free days in Western Australia radon is thick on valley floors in those areas that have it in the still of the mornings, and gone later in the afternoon as the winds increase and clear the heavy ground hugging gases.

I would suggest that radon is a function more of geology and topography .. modified in variance by air pressure and wind conditions.

In any case, thank you for your response, I look forward to any further thoughts you may have, and, in closing, you may enjoy this entertaining A0 wall map [5] .. or perhaps the larger raw datasets (also available) [6]

[1] https://inis.iaea.org/collection/NCLCollectionStore/_Public/...

[2] https://www.theguardian.com/australia-news/2023/jan/28/missi...

[3] https://www.aap.com.au/news/search-for-radioactive-capsule-i...

[4] https://ww2.health.wa.gov.au/Media-releases/2023/January/Sea...

[5] https://ecat.ga.gov.au/geonetwork/srv/api/records/a05f7892-f...

[6] https://www.ga.gov.au/scientific-topics/disciplines/geophysi...

Anyway, you’ve clearly got skin in the game in this argument, Aussie. I wonder what your role is: an interested observer or an involved party?

As such it radiates equally in all directions.

Of course a manufacted Source(TM) for industry might actually be a slug of doped Cs-137 surrounded by a lead casing with a window to direct emissions.

This still leaves us talking about the actual source of the radiation which emits equally in all directions .. but being blocked in some directions by either a casing or perhaps the earth below, a pool of water, etc.