Q:
Been looking for a calculator like this to help me calibrate some of my
meters:

http://www.radprocalculator.com/Gamma.aspx

Probably been mentioned here many times before, but I am wondering about its
accuracy?
I have a 1 uC disc source of Cs-137 which the calculator says should produce
0.5 mR/hr at 1" away (I used the gamma dose rate & shielding
calculator). Can I count on this calculated value for a rough calibration
for a meter?

Thanks,

A:

The calculator is very accurate - I use it all the time.That site has many
other rad calculators that are useful.

As far as expecting to read that exact figure on a meter though, it's just a
bit more complicated than that.

Let me say that a Gamma Range, fuelled with Cs-137, is the standard way to
rad calibrate radiological instruments, so the idea is sound.

In practice, the farther away from the source, the more accurate the reading
will be, as long as the rad field is strong enough to give a good 2/3 scale
reading. The reasons for this are twofold. First, because of the inverse
square law, the fact that halving the distance results in 4X the radiation,
or conversely,
doubling the distance yields 1/4 of the rad field, it is easy to see that
any minute errors in measurement at 1" are going to lead to a large
difference in readings. At a greater distance say 5 FEET, a minor difference
of a fraction of an inch are statistically unimportant.

Be aware that the measurement is from the surface of the rad source to the
CENTER of the detector probe volume, not the surface of the probe At short
distances
errors will creep in if you don't.

Now consider the second important factor: Geometry.
In this context "geometry" is referring to the shape of the radiation field
compared to the size of the probe.

Radiation is considered as coming from the point source in all directions,
such that it would describe a sphere. In mathematical terms, this would be
called 4 pi steradians.
Naturally most probes can only intersect a portion of that sphere. If the
portion that strikes the probe is as large as the probe or larger, a
realistic
measurement can be taken limited only by the physics of the detector system
( these limitations and variables are many- each subjects of a discussion
alone).
In any event, since at best it can only see 1/2 of the sphere, probes are
said to be responding to 2 Pi ( with some probes, the sample gets inserted
INSIDE for a true
4 pi reading). Such a situation must be accounted for in the electronics or
on paper when trying to estimate TOTAL ACTIVITY. In simple terms,
detected activity= (total activity / 2) * probe efficiency, assuming the
whole probe is illuminated. Rather than clutter up this post with trivial
matters, here is a
reference that explains the steradian thing:
http://en.wikipedia.org/wiki/Steradian

Most measurements are based on the rad source being 10X the probe's longest
dimension away from the probe ( 30 inches for a 3 inch probe sensitive
volume).

At closer distances, errors creep in rapidly.Volts or eV.

Let's recap the Gamma energy issue as it is very important to understand the
part played by energy level in detection in general. *Activity* is the term
we use
to indicate the AMOUNT of radiation from a particular source and the unit is
Curies (Ci) or MicroCuries (uCi). *Energy* is the term that indicates how
STRONG the radiation is. Units here are electron-Volts. Usually we are
dealing with thousands of electron-Volts, abbreviated keV. A million
electron-Volts is called MeV.
So consider a single test disc of 1 uCi activity from Cs-137. We know the
energy level is mainly 662 keV. Now add another identical disc. The energy
level is still 662 keV, but there is now twice as much ACTIVITY. **MORE OF A
SOURCE DOS NOT MAKE THE RADIATION STRONGER. It makes more of the same
strength radiation. It will not penetrate a shield any farther than a
smaller sized source, but there will be more gets through a given shield
because here is more going in the other side.

For any isotopic energy other than Cs-137, a correction factor is needed for
most probes. The mechanics of this phenomena are wrapped up in the physics
of how
a probe detects Gamma radiation. When Gamma radiation energy is too weak,
that is of low energy, it will not penetrate into the sensitive part of the
probe, being
blocked by the materials from which the probe is made. Once that energy
level is raised to the point it CAN penetrate, detection starts to take
place quite readily.
As the energy rises, so does the efficiency of the detection- UNTIL the
energy gets to a point that it starts to penetrate the probe entirely. The
detection efficiency
curve is anything but a straight line. Each type of probes has a different
response curve, as subject of yet another discussion. These curves are
represented on the Ludlum website for most of their probes, and some are
posted in the FILES section of
http://tech.groups.yahoo.com/group/GeigerCounterEnthusiasts/

The practitioner must apply the correction factor to the measurements as
needed, necessitating that one knows the nature of the radiation to
calculate a true rad measurement from the meter indications. With mixed
energy sources such as natural background radiation, the energies are from
all kinds of different decay
products in the environment, each being affected by local shielding
differently, and at the same time, each causing the probe to respond
differently. *Quantitative Measurements* as opposed to *detection* are a
real moving target.

Have fun

Geo