Radiation Detection Probes and Their Dial Scales> A Tutorial by Geo

Pay attention to the Gamma Sensitivity numbers in the specification sheets and charts. This figure gives a clue as to how sensitive a certain tube is compared to another tube or probe. The number indicates how many pulses you would get from a uniform flux of Cs-137, in counts per minute/per mR/H. Each probe must use a dial scale that is correctly delineated for the CPM/mR/H of that probe. Only LUDLUM MEASUREMENTS makes dial scales that are easily changed to match their respective probes. NOTE: The mR/H scale is ONLY accurate when measuring Cs-137, OR when using an energy compensated probe (44-38).

                       
For all probes except the few specifically labeled “Energy Compensated”, the other factor is Gamma Energy Response> different tubes and probes  will respond to varying energy levels according mainly to he construction materials used, and volume and pressure of fill gas or crystal (size of probe). In general, Low Energy Gammas (LEG) must be of sufficient strength (meaning energy level, not number of disintegrations) to penetrate the housing material. A Z number is used to indicate density of any material, based on atomic makeup. Some probes utilize LOW Z windows to allow in extra low energy rays and particles.

Once inside the tube, lower energy Gammas are much more likely to cause an interaction, and therefore be counted. At some point as the energy level increases, the ray will simply pass out of the tube and not be counted. These facts account for the whipsaw shape of the energy response curves of all GM tubes. External filtering may be applied to compensate for this non linear effect, resulting in a probe that is called" energy compensated". Be aware that this procedure knocks all the response down to the lowest level, and that although now nearly perfectly linear, such a probe will give lower reading than you may be used to from the more common "energy dependant" probes.

Making a rough estimate of activity may be found by applying this formula:
@1 meter 1Ci= .381 R

where 1uCi=10^-6 Ci

and using the inverse squared law:
@ 1/2 meter = X4
@1/4 meter = X 16
@ 1/8 meter= X64
@ 1/16 meter = X256
etc.

1 uCi is always equal to 3.7 X 10^4 DPS (disintegrations per second) or 2.22 X10^6 DPM no matter what type of radiation is involved.
http://www.sizes.com/units/curie.htm
http://www.radcon.arizona.edu/training/RSPC-CH.pdf

When the term 4Pi is used, it refers to disintegrations in all directions, as in a sphere. Most probes can only see from one
direction and as such are 2Pi (1/2 of a sphere). GEOMETRY is the term used to indicate the area that the radiation fills in relation to the probe. Technically it is the angle subtended by the probe.
Best geometry is achieved if the probe is 10X it's own diameter away from the source.
4Pi or near 4Pi can be achieved with hollow probes (as in WELL probes) where the radioactive sample is placed inside. Liquid scintillators are also 4Pi, as the sample is inside the liquid.

 

 

   

202-330
0-4k cpm; 0-2 mR/hr
For Model 44-7 

 

 

202-241
0-2 mR/hr; 0-2.4k cpm
For Model 44-6; 44-38

 

 

 

 

 202-654
0-50 mR/hr; 0 - 8.5k cpm
For Model 44-2

 

 

 

 

 

202-717
0-5 mR/hr
For Model 44-10

 

 

202-212
0-5 mR/hr; 0-3500 cpm
For Model 44-3(I-125)

 

LOW ENERGY GAMMA (LEG) Probes have a thin crystal, making them more like"non-high energy detectors".

Since low energy Gammas and X-Rays are absorbed in the first 1/100th of an inch in NaI(Tl), there is no need to make the crystal any thicker than that.

Without a thick crystal, high energy rays are not well absorbed, therefore add little to the desired signal.

In addition most LEG probes incorporate some sort of thin entrance window, making it easier for the LEG to penetrate into the crystal.

 

 

 

George Dowell

New London Nucleonics Laboratory

Copyright © Viscom Inc. 2007

The treatise may under no circumstances be resold or redistributed in either printed, electronic,

or any other forms, without prior written permission from the author.

Comments, criticism and questions will be appreciated and may be directed to

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Hello Folks!

This is an explanation of how nuclear decay happens, and where the
alpha, beta and gammas we all love to measure come from.

For all intents and purposes, nuclear decay is always by a particle
exchange mechanism. The nucleus either emits or captures a partial.
Generally, this is either by:

1) Alpha decay - the nucleus undergoes what amounts to a very uneven
spontaneous nuclear fission, and emits a He nucleus (2 neutrons, and
2 protons). It steps 2 elements down the periodic table, and loses 4
mass units. This is the alpha decay we all talk about, though I've
read that many other forms of alpha decay do exist in some more
exotic isotopes, where the emitted "alpha" is other than a helium
nucleus, but is some other light nucleus. We're pretty unlikely to
encounter this in the home lab, methinks.... In my mind, any
spontanious fission is actually an exotic alpha decay.

2) Beta- decay - the nucleus emits an electron, and thus the nucleus
charge goes +1, and the nucleus "climbs" one step up the periodic
table. The beta (electron) is emitted by a neutron in the nucleus,
and in doing so, the neutron becomes a proton. The mass is
essentially the same, but the charge is now different

3) Beta+ decay - The nucleus emits a positron (Anti-matter in the
Home Lab!), or a positively charged "electron". Na22 is one such
isotope. In this case, the nucleus charge goes -1, but again the
mass is essentially the same.

4) Electron Capture (EC) - another rather exotic process, basically
the reverse of beta decay. A proton in the nucleus absorbs an
electron and becomes a neutron. Mass is the same, charge is -1, as
in beta+ decay.

All these decay modes may leave the nucleus in an upset state. Like
a Jenga tower with bricks removed from below, it's unstable. As
it "collapses" to a more stable state (the jenga tower sees this as
a pile of rubble), it has to release energy. That energy is
typically a Gamma Ray, a high energy photon. Gammas are simply a
result of the new nucleus relaxing and coming to terms with its new
form.

In the case of the natural decay chains, each step is by one of the
processes above (generally B- or Alpha). After each step, a gamma
may be emitted as the new nucleus relaxes. Decay chain charts don't
always list these gammas because they're not relevant to the actual
changes. But, if you look at each isotope in the decay chain, look
it up in Wiki or whatever you like, you'll see what gammas it
typically emits in its decay.

As an aside, the difference between Gammas and X rays: To your
geiger counter, there is NO DIFFERENCE! The only difference is that
the gamma is made by a nucleus performing an energy balancing act
(as I described above), while an X-ray is the result of the atom's
electrons moving to a lower energy state. So Gammas come from the
nucleus, and are a nuclear event, while Xrays are from the electron
cloud, and are an electronic event.

Hope it helps...

J

At a distance of 2.5 cm, bubs are running 4 or 5 per day.

Bubbles captured from exposure to the DU target while it was being bombarded with slow neutrons.

Above is a test showing fast neutrons captured right next to the bare source, that is no

modereator. There were 8 bubbles left in the BTI from previous tests, and they should be

disregarded in the total count here. These include those two large bubbles.

Greetings group,

my name is Kevin,

I have had an interest in mineralogy for some time, and have found
an old friend I used to toke around with me years ago.

Something that might interest the mineralogist in you is a scan of a
field pocket manual I obtained from the Australian Atomic Energy
Commission (A.A.E.C.)in the mid seventies.

It is entitled Selected Uranium Mineral Specimens and is a pdf/zip
download.

http://www.vk3ukf.com/vk3ukf_files/eBook/SUMS.htm

I guess it's an eBook then.

There's probably some other stuff there, that some of you would get,
you'll have to look at the menu on the main page.

http://www.vk3ukf.com/index2.htm

All the best.
Kevin, VK3UKF.

Hello,
Geo pointed me to this Yahoo group and mentioned there was some
previous work done on granite by this group.   I looked in the Files
section but did not see it.

Is this info still available?

Thanks,
Al

Hello, I have some Dosimeters that can be seen in the group's photos
section at;

http://tech.ph.groups.yahoo.com/group/HomeRadLab/photos/view/6a9c?b=1

I have lost 2 of my low dose Quartz Fibre Dosimeters and I am looking
to try and find low cost replacements for them. They are not used
professionally, so I'm not looking to pay professional prices.

The 2 Dosimeters in the photo have ranges 0 - 200 ROENTGENS, the
other is 0 - 150 r (which I assume is the same as 0 - 150 ROENTGENS).

The ones I lost were 0 - 0.2r, again I am guessing that's 0 - 200
milliroentgens? Please correct me if I am wrong here...

I al looking to purchase at least 1 low range/dose Quartz Fibre
Dosimeter(s), if anyone can help in the supply of 1 or 2 that are of
the same design as the ones in the photo so that they can be charged
using my original Stephen Universal Quartz Fibre Dosimeter charger
(as per the photo) and their range is 0 - 0.2r or less.

Happy new year and thanks in anticipation.
Keith Watt RN (Rtd.)

Geo:

Yes, it is not a good idea to take apart smoke Detectors, a recent
site suggested using the source for a homemade Spinthariscope - since
it is an Alpha emmitter, failing to mention its Gamma components...
(Australian site, I think)


Anyway, the Radioactive  Boy Scout was Sentenced to 90 days

http://www.foxnews.com/story/0,2933,299362,00.html

http://en.wikipedia.org/wiki/David_Hahn

History:
http://www.abc.net.au/science/k2/moments/s595641.htm

http://www.dangerouslaboratories.org/radscout.html

Enjoy,

John-

Hi,
     Dug out my 20 year old self luminous exit sign and found it still
luminescent and fully quiet - no counts of any kind. It's a totally
sealed unit, appears to be seam welded plastic.
     Does this unit have and experimental use? Or just a curiosity?

Thanks

A brief list can be found here:

http://tubecollector.org/documents/radioactive.htm

Not all tubes are still radioactive now, if they have short lived isotopes.

Thorium charged tubes are just barely above background.  But the ones with
radium are quite hot and will be for thousands of years.

Steve.




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