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Re: [Phys-L] Half-Life measurement


Excellent questions. A couple of pictures will answer most of those:
We've got three inches of steel shielding on all sides. Photos have the
access door open and the top removed. Those would be in place when taking
data, naturally. Students measured 0.3 counts per second for their
background. That's less than half of the 0.7 number you found. So the
shielding is doing something.

More than wind, atmospheric pressure, or humidity, the largest influence on
radon levels is probably whether or not the building air handling system is
running. They like to shut it off overnight.

The table is a stone composite. The walls are cinder block. The floor is
concrete. All of these certainly have some level of activity that's higher
than you would find in a wood framed house.

Cosmic rays would likely account for less than 0.1 counts per second in our
And that varies sinusoidially at the 1% level over a 24 hour time period.

Ok, so the next step here is to run a long-term study of background
activity using this geiger tube.


On Wed, Oct 13, 2021 at 11:27 AM John Denker via Phys-l <> wrote:

In case it wasn't obvious:

Using a *cumulative* counter and a clock is a Good Thing,
much preferable to using a resettable incremental counter
and an interval timer. Most of the lab procedures I've
seen call for the latter, which causes loss of information.

-- If you have the cumulative data and want the incremental
data, you can always get it by subtraction.

-- If you have only the incremental data for "selected"
intervals, you have no idea what was going on between
those intervals.

The cumulative data allowed me to plot both the Laurels and
the Hardys, as discussed in the previous message. It turns
out that the Hardys contain a lot of good information. If
you thought of the Laurels as "the" readings, all the Hardy
information would get thrown away.

It's often amazing how small details in the lab technique
can have a large impact on the outcome. I've seen patent
lawyers ask: You succeeded, but what did you do differently
from the thousands of guys who tried the same thing and
failed? Then it takes a year to answer the question.

Unrelated issue: On 10/12/21 11:06 PM, Paul Nord wrote:

My concern about modeling the background is not about what the value
is. Rather, I’m doubtful that the background is constant over
several days. The sensitivity of the detector might also not be
stable over the long time. That would show up as very much the same

Do we know where the background is coming from?
What's the physics?

My ill-informed thoughts include:

*) I assume the apparatus is well shielded against alphas
and betas, so we need to hunt for gamma sources, which
are a lot harder to shield against.

*) Even if the background is not variable, it's high enough
to be annoying. I've seen lab writeups on the web that report
a lower background, around 0.7 events per second.

A variable background is even worse. So we need to hunt for
variable as well as non-variable sources.

*) There's a lot of radon in Porter County. The indoor action
limit is 4pCi per liter, which is .15 Bq per liter, i.e. .15
decays per second per liter, which seems like a lot. The
radon decay chain produces some gammas.

The indoor amount of radon might depend on which way the wind
is blowing and other hard-to-control factors.

*) People are full of potassium, which emits gammas. However
my estimates suggest this is not enough to be significant.
Almost, but not quite. So I don't think people near the
apparatus are a problem.

*) The cosmic ray secondary flux is variable, but it's not
super huge. I haven't found any good data on how much of
it is gammas.

*) I hate to ask dumb questions, but is there any chance your
lab bench has a beautiful granite countertop?

Granite is notoriously rich in gamma emitters.

*) Is there any chance your shielding materials are slightly
radioactive? Probably not, but stranger things have happened.
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