[ RadSafe ] Silly things to know about detection systems and HP ops. other instances
blreider at aol.com
blreider at aol.com
Fri Oct 1 13:08:06 CDT 2010
Dr. P,
I thought the PMT detector string was interesting so I hope people like this one. I don't have as much lab experience as many here but I did work with equipment for a while and some of what I think are good practices and lessons learned are listed. Of course there are many ways to do things.
Some of the statistics computer codes on instruments self adjust the sigma boundaries every so many pieces of QC count data. If you have small statistical shifts each day the lab can easily miss big problems over time unless there is diligent oversight. Absolute boundaries of acceptable range can be better to use than statistical boundaries. You do have to adjust absolutes to account for decay of check source, etc.
Statistical indicators and sigma values set right after calibration (you know, you do the 30 or so short counts in a row with the check source) might be too stringent if the lab environmental conditions don't reflect the variety of conditions you have during normal counting hours. So one might think that one in 20 counts might be outside the sigma boundaries but the system goes out more often.
Also some of the regulators and programs here in the US require recalibration of spectroscopy systems annually. A calibration is only a mathematical model, so if the system runs well and has a robust QA program (key) there is no need to change the model.
The women in their skirts was a good story I had never heard it. Back then the cloth would have been non-synthetic too.
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: Fri, Oct 1, 2010 9:56 am
Subject: Re: [ RadSafe ] Silly things to know about detection systems and HP ops. other instances
Barbara Reider,
Good idea! you have a good collection already
ther instances;
I used to regularly record the background counting rate of a sensitive
hole-body monitor - sort of control chart. The chart had two parallel lines to
ndicate plus or minus three standard deviations.
After a while I observed that the " stable" readings showed a clustering
endency- though all readings were well within the three standard deviation
imit. We observed it for a week. Almost simultaneously, patients started
omplaining 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
eversed. The ventilator was drawing air from the shielded room rather than
riving air into the room.We could see dust particles at the edges of the
assively shielded door. Believe it or not, this was reducing the airborne
atural radioactivity seen by the eight Na I(Tl) detectors causing a tiny
eduction in background counting rates and causing a downward shift in reading
The staff corrected the error in the wiring of ventilator unit; background
eading increased. The system malfunctioned for several days.
have published this observation in " Nuclear Instr. and Methods" a few years
go
------------------------------------------------------------------------------------
Immediately after the accident at the Three Mile Island nuclear power station,
he station management outsourced whole body counting of some persons (I think
embers of the public). The company who operated the mobile whole-body monitors
eclared that many of them have radium contamination! They were actually
easuring the radon decay products deposited on the apparels of the persons. If
recall correctly the Kemeny Commission appointed by the President made some
emarks on the shoddy incident.
Regards
arthasarathy
_______________________________
rom: "blreider at aol.com" <blreider at aol.com>
o: radsafe at health.phys.iit.edu
ent: Fri, 1 October, 2010 11:34:25
ubject: [ RadSafe ] Silly things to know about detection systems and HP ops.
r. Parthasarathy,
I do hope you and others add to this, I think it will be fun and useful to
ome. I liked the story you posted, it is so true that some dumb thing
verlooked can cost time and money. So I thought it would be useful to start a
tring where people can post dumb things they have seen other people do. Of
ourse none of us have done any of these so there is no shame in posting!
ere 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
t
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
hat
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
round
you that you might not see.
One nuclear plant I visited years ago had periodic larger than expected Cs-137
eaks on their NaI Whole Bdy Counter (WBC). These showed up during times when
hey were counting people so they were thought to be + counts. It turned out
he facility had a sludge tank behind a half wall in the next building and when
he 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
he building next door.
eople doing other tests at the detector nearby can have sources nearby that
lter your detection system's background.
amma spec background data can be impacted by interferences by Compton edges,
ail ends of other peaks, etc. This can result in a false lowering of the
ctual 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
hile, then during an interview the worker couldn't stop picking his tooth with
is finger. He had not inhaled or ingested the radio nuclide, he had
ontaminated his tooth and was slowly entering his GI tract.
f 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
as been removed from the detector.
Radon from well-water can cling to clothing, this is especially a problem for
ortal 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.
oing things you shouldn't be doing on the job in a contaminated area is not a
ood idea. The investigation can be embarrassing.
he partner of someone who has a medical test such as a heart stress test can
ransfer detectable amounts of material to the plant worker through fluids.
f an area air sampler on one side of the room has highly elevated levels, and
he area sampler on the other side of the room does as well, perhaps it is time
o 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
s
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
vening
checks might not catch all the shifts. Ge detection systems also can have
emp.
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
>us.
>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
dispenser.
> If you have a lot of material, 10 half lives may not be the amount of time
ou
need to reduce the material to below regulatory levels.
> Large area detectors need to be efficiency calibrated for a geometry that
akes
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
>release.
> 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
>dose.
> 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.
t is kind of fun to think of the "gotchas" and this might make a nice little
ist if you all put your observations in. I should have started earlier, it is
ow so late!
Thanks
arbara Reider, CHP
-----Original Message-----
rom: parthasarathy k s <ksparth at yahoo.co.uk>
o: The International Radiation Protection (Health Physics) Mailing List
radsafe at health.phys.iit.edu>
ent: Thu, Sep 30, 2010 10:44 pm
ubject: Re: [ RadSafe ] low energy gamma-spikes pranks done with
hotomultiplier tubes! Blocked air flow causes problem
ear Dr Dixon,
n our counting laboratory in Leeds we found an inexplicable shift of a few
annels in the case of a sensitive gamma spectrometer. The multichannel
alyser and associated instrumentation belonged to one company (Nuclear
odes??) whereas the detector was from Nuclear Enterprises. Both companies
fended their turf.
e recorded room temperature continuously. The ambient temperature in the sub
sement laboratory was strictly controlled within plus or minus a few degrees.
spent better part of a month trying to figure out the cause of shift. The
ason was pretty simple. Some one has inadvertently kept a folder (the typical
nctional one distributed in conferences) over the electronic instruments
ereby blocking the air flow.
ere was another instance in which an instrument behaved quixotically because
e fibrous filter which removes dust got clogged over the years. We may not get
uch experience from text books
egards
arthasarathy
_______________________________
om: "Dixon, John E. (CDC/ONDIEH/NCEH)" <gyf7 at cdc.gov>
: The International Radiation Protection (Health Physics) Mailing List
adsafe at health.phys.iit.edu>
nt: Thu, 30 September, 2010 23:05:54
bject: Re: [ RadSafe ] low energy gamma-spikes pranks done with
otomultiplier tubes!
ith regard to the energy spectrum shift, I have experienced such a shift in
lid state detectors due to extreme variances in environmental temperature (the
emperature of the area being surveyed). I haven't heard of the 'whisker'
enomenon.
ohn E. Dixon
_______________________________
rom: radsafe-bounces at health.phys.iit.edu on behalf of parthasarathy k s
nt: Thu 9/30/2010 12:34 PM
: The International Radiation Protection (Health Physics) Mailing List
bject: Re: [ RadSafe ] low energy gamma-spikes pranks done with
otomultiplier tubes!
I recall that in our formative years in the Bhabha Atomic Research Centre, we
ed to all sort of crazy things with photomultiplier tubes. In our enthusiasm
e of my colleagues used a magnet of considerable strength and virtually
splaced the electrode of a PM tube with disastrous consequences!
ndeed we studied the response by removing the electrostatic shields. We also
udied influence of earth's magnetic field; if I recall correctly it was
gnificant for large area photomultiplier tubes.
hile on this, let me describe a very interesting experience we had while using
rge area photomultipliers.
he Department of Medical Physics, University of Leeds had an understanding with
e Nuclear Enterprises by which the company used to pass on low background
otomultipliers with sodium iodide (Tl) crystals as integral line assemblies
r studies. While working with one such units our routine was to record Cs-137
.66 MeV) gamma line and TL-208 (2.61 MeV) gamma line at the beginning. One day
found that both these lines have shifted considerably down. It appeared that
ere was some change in the internal electrode structure which led to this.
well experienced member of the technical staff told us that he had found such
event earlier; he went through his old records and located a remedy.We found
at in large area PM tubes small whisker like structures can develop and work
a short circuit lowering the voltages between electrodes. The impact of this
ll be considerable if it happens at the initial stages. The remedy was to send
ery low voltage high current pulse between the suspected electrodes and burn
f the whisker. Once this was done the gamma lines appeared in the same
annels of the multichannel analyser.
Regards
rthasarathy
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