[ RadSafe ] Silly things to know about detection systems and HP ops. other instances

parthasarathy k s ksparth at yahoo.co.uk
Fri Oct 1 07:01:16 CDT 2010

Barbara Reider,

Good idea! you have a good collection already
other instances;

I used to regularly record the background counting rate of a sensitive 
whole-body monitor - sort of control chart. The chart had two parallel lines to 
indicate plus or minus three standard deviations.

After a while I observed that the " stable" readings showed a clustering 
tendency- though all readings were well within the three standard deviation 
limit. We observed it for a week. Almost simultaneously, patients started 
complaining that they felt very stuffy in the shielded room.

Over a week end, the electrical maintenance staff after dusting and cleaning the 
room's ventilation unit reversed the wiring inadvertently; air flow got 
reversed. The ventilator was drawing air from the shielded room rather than 
driving air into the room.We could see dust particles at the edges of the 
massively shielded door. Believe it or not, this was reducing the airborne 
natural radioactivity seen by the eight Na I(Tl) detectors causing a tiny 
reduction in background counting rates and causing a downward shift in reading 
.The staff corrected the error in the wiring of ventilator unit; background 
reading increased. The system malfunctioned for several days.
I have published this observation in " Nuclear Instr. and Methods" a few years 

Immediately after the accident at the Three Mile Island nuclear power station, 
the station management outsourced whole body counting of  some persons (I think 
members of the public). The company who operated the mobile whole-body monitors 
declared that many of them have radium contamination! They were actually 
measuring the radon decay products deposited on the apparels of the persons. If 
I recall correctly the Kemeny Commission appointed by the President made some 
remarks on the shoddy incident.


From: "blreider at aol.com" <blreider at aol.com>
To: radsafe at health.phys.iit.edu
Sent: Fri, 1 October, 2010 11:34:25
Subject: [ RadSafe ] Silly things to know about detection systems and HP ops.

Dr. Parthasarathy,

I do hope you and others add to this, I think it will be fun and useful to 
some.  I liked the story you posted, it is so true that some dumb thing 
overlooked can cost time and money.  So I thought it would be useful to start a 
string where people can post dumb things they have seen other people do.  Of 
course none of us have done any of these so there is no shame in posting!    
Here are some obvious time eaters I observed:

>  Make sure the instrument is plugged in.  I was in an MRI machine years ago in 
>NY and the tech couldn't get the machine going.  I finally said "did you plug it 
>in?" and it turned out the power cord was frayed.  Eaten by rats per the tech - 
>not a warm fuzzy for me in a medical facility.  But he got the machine going, I 
>had shoulder surgery and lived to windsurf again!

>  Noble gasses can cling to people and clothing and can move around a plant that 
>way.  One hears of power plants that spend a lot of money because the staff 
>didn't plan on radon from homes and containment maintenance work being detected 
>one minute and not being around later on.  Plastics and synthetic cloth are big 
>culprits for attracting noble gasses.  

>  If your lab has efficiency problems with gamma emitter that are commonly all 
>over the place at the plant check to find out what processes are going on around 
>you that you might not see.  

One nuclear plant I visited years ago had periodic larger than expected Cs-137 
peaks on their NaI Whole Bdy Counter (WBC).  These showed up during times when 
they were counting people so they were thought to be + counts.  It turned out 
the facility had a sludge tank behind a half wall in the next building and when 
the sludge was stirred occasionally the cesium would rise above the wall level 
(I know it is a salt but there apparently was some precipitate) and shine into 
the building next door.
People doing other tests at the detector nearby can have sources nearby that 
alter your detection system's background.
Gamma spec background data can be impacted by interferences by Compton edges, 
tail ends of other peaks, etc.  This can result in a false lowering of the 
actual dose you are trying to evaluate

>  Bioassay requires observation.  

Los Alamos HPs gave a talk years ago about a worker whose urine levels of Pu-239 
were elevated and didn't reduce over time.  They couldn't figure out why for a 
while, then during an interview the worker couldn't stop picking his tooth with 
his finger.  He had not inhaled or ingested the radio nuclide, he had 
contaminated his tooth and was slowly entering his GI tract.
If the worker in your WBC at a nuclear plant has Cs-137 and Co-60 levels about a 
tenth of the amounts in your check source, check to see that the check source 
has been removed from the detector. 

Radon from well-water can cling to clothing, this is especially a problem for 
portal monitors in the morning in areas of course with high uranium or radium in 
the environment. 

Mouth pipetting is not the best of all methods.
Doing things you shouldn't be doing on the job in a contaminated area is not a 
good idea.  The investigation can be embarrassing.
The partner of someone who has a medical test such as a heart stress test can 
transfer detectable amounts of material to the plant worker through fluids.
If an area air sampler on one side of the room has highly elevated levels, and 
the area sampler on the other side of the room does as well, perhaps it is time 
to check the intakes of all the people working between the two samplers.

>  Old media can lose data with temperature changes.  Old computers were 
>notorious for problems with data storage as disks expanded and contracted with 
>high summer temperatures.  I think this is less of a problem now that storage is 
>more compacted and on different types of materials. 

>  Of course you all know that NaI detectors have peak energy shifts with 
>temperature change.  If your lab has changes during the day, morning and evening 
>checks might not catch all the shifts.  Ge detection systems also can have temp. 
>shifts however for larger changes in temperatures.  Always a good practice to 
>identify all peaks in the spectrum. K-40 is a good check peak we all carry with 

>Iodine plates out on long cold collection tubes so what is seen at the far 
>detection end may be under detected.

>Plutonium and other radio nuclides can plate out on container walls, which can 
>reduce the amount in solution and alter results of a test.  

>Tritium moves.  Leaves.  Plastic and rubber O-rings help this along.

>Mixing Am-241 and beryllium can increase neutron levels.

>Very high levels of alpha emitters can break down plastic bags and floor covers 
>faster than expected, of course the creep of material could also be 
>contamination the usual way.  

> It is not good to have the exhaust from one lab placed right at the intake to 
>another lab.

> Cool old stuff like scotch tape dispensers used depleted U as a weight in the 

>  If you have a lot of material, 10 half lives may not be the amount of time you 
>need to reduce the material to below regulatory levels.

> Large area detectors need to be efficiency calibrated for a geometry that makes 
>sense and is repeatable.  If you are using a large area detector to look for 
>small particles you need to know what the bounds of the results will be with 
>position, size and density of the subject radio nuclides.

>Sky shine can affect boundaries of facilities with appreciable amounts of 
>penetrating radiations.  Even more in warm climates with no ceiling shielding.

> Release of uncontaminated feral cats and other wild animals caught at a highly 
>contaminated site to a nearby location will only cause further contamination 
>spread.  Get the local Heath Dept involved or at least decon the animals before 

> Some oil, gas and water facilities can have plate out of thorium isotopes in 
>addition to radium.  Thorium can be  more of a problem than radium for internal 

> Set points often need to be set considering the hardest to detect radio 
>nuclides rather than the easy to detect ones.

>QA should be done at the final part of the process after all processes are 
>finished bu before products are released.  An analyzed in time to recall a 
>product before it is released.  Especially if that product is a medical isotope 
>to be injected into a patient.

It is kind of fun to think of the "gotchas" and this might make a nice little 
list if you all put your observations in.  I should have started earlier, it is 
now so late!

Barbara Reider, CHP  

-----Original Message-----
From: parthasarathy k s <ksparth at yahoo.co.uk>
To: The International Radiation Protection (Health Physics) Mailing List 
<radsafe at health.phys.iit.edu>
Sent: Thu, Sep 30, 2010 10:44 pm
Subject: Re: [ RadSafe ] low energy gamma-spikes pranks done with 
photomultiplier tubes! Blocked air flow causes problem

Dear Dr Dixon,
In our counting laboratory in Leeds we found an inexplicable shift of a few 
hannels in the case of a sensitive gamma spectrometer. The multichannel 
nalyser and associated instrumentation belonged to one company (Nuclear 
iodes??) whereas the detector was from Nuclear Enterprises. Both companies 
efended their turf.
We recorded room temperature continuously. The ambient temperature in the sub 
asement laboratory was strictly controlled within plus or minus a few degrees. 
e spent better part of a month trying to figure out the cause of shift. The 
eason was pretty simple. Some one has inadvertently kept a folder (the typical 
unctional one distributed in conferences) over the electronic instruments 
hereby blocking the air flow.
here was another instance in which an instrument behaved quixotically because 
he fibrous filter which removes dust got clogged over the years. We may not get 
such experience from text books

rom: "Dixon, John E. (CDC/ONDIEH/NCEH)" <gyf7 at cdc.gov>
o: The International Radiation Protection (Health Physics) Mailing List 
radsafe at health.phys.iit.edu>
ent: Thu, 30 September, 2010 23:05:54
ubject: Re: [ RadSafe ] low energy gamma-spikes pranks done with 
hotomultiplier tubes!
With regard to the energy spectrum shift, I have experienced such a shift in 
olid state detectors due to extreme variances in environmental temperature (the 
temperature of the area being surveyed). I haven't heard of the 'whisker' 
John E. Dixon
From: radsafe-bounces at health.phys.iit.edu on behalf of parthasarathy k s
ent: Thu 9/30/2010 12:34 PM
o: The International Radiation Protection (Health Physics) Mailing List
ubject: Re: [ RadSafe ] low energy gamma-spikes pranks done with 
hotomultiplier tubes!

I recall that in our formative years in the Bhabha Atomic Research Centre, we
sed to all sort of crazy things with photomultiplier tubes. In our enthusiasm
ne of my colleagues used a magnet of considerable strength and virtually
isplaced the electrode of a PM tube with disastrous consequences!

ndeed we studied the response  by removing the electrostatic shields. We also
tudied influence of earth's magnetic field; if I recall correctly it was
ignificant for large area photomultiplier tubes.

hile on this, let me describe a very interesting experience we had while using
arge area photomultipliers.
The Department of Medical Physics, University of Leeds had an understanding with
he Nuclear Enterprises by which the company used to pass on low background
hotomultipliers with sodium iodide (Tl) crystals as integral line assemblies
or studies. While working with one such units our routine was to record Cs-137
0.66 MeV) gamma line and TL-208 (2.61 MeV) gamma line at the beginning. One day
e found that both these lines have shifted considerably down. It appeared that
here was some change in the internal electrode structure which led to this.
A well experienced member of the technical staff told us that he had found such
n event earlier; he went through his old records and located a remedy.We found
hat in large area PM tubes small whisker like structures can develop and work
s a short circuit lowering the voltages between electrodes. The impact of this
ill be considerable if it happens at the initial stages. The remedy was to send
very low voltage high current pulse between the suspected electrodes and burn
ff the whisker.  Once this was done the gamma lines appeared in the same
hannels of the multichannel analyser.


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