Fukushima radiation detection and measurement

[Bob S]

(This a new thread as proposed by Borek post #2349)

To all: please don't continue discussion on the radiation/health issues in [Japan earthquake-nuclear plants]

Astronuc will the radiation meters the workers have detect if they walk near neutron beams? What is your opinion of the neutron beams that was reported earlier at the plant ?

Neutrons and neutron "beams" are very hard to detect with portable instruments. The most common portable instruments are gas proportional counters using thermal neutron capture in BF₃ (boron tri-fluoride) or He³ gas. Proportional counters saturate (paralyze) at high counting rates, so are not good in high radiation areas. Sodium iodide is not a good neutron detector. See

http://www.orau.org/ptp/collection/proportional counters/bf3info.htm

http://www.gepower.com/prod_serv/products/oc/en/oilfield_technology/drilling_measurements/he3_neutron.htm

I have used GM tubes wrapped with thin silver foil (Ag¹⁰⁷ activation with 2.3 min lifetime) to detect pulsed neutron beams. Tissue-equivalent (Shonka) ionization chambers with suitable neutron moderator and gas (ethene) give a good Sievert (rem) response
to mixed (beta gamma neutron) response, even in high radiation fields (when properly designed). See

http://www.orau.org/ptp/collection/ionchamber/shonkatissueequivalent.htm

High energy neutrons produce proton recoils in a hydrogenous gas (like ethene or ethane) in an ion chamber.

Focusing neutrons is like herding cats. Neutrons are produced isotropically. Neutrons diffuse through shielding, and may leak through cracks, but since they are uncharged, they cannot be focused.

Bob S

 

[Bob S]

This chart from

http://research.uncc.edu/sites/research.uncc.edu/files/media/IsotopeSheet.pdf

gives a good relation between radioactive contamination (milliCuries) and radiation dose (millirem per hour) for many isotopes.

Bob S

 

[robinson]

Fascinating. I was just looking at the Wiki article
http://en.wikipedia.org/wiki/Acute_radiation_syndrome#Exposure_levels
and wondering why no mention of skin problems from radiation, when they got their feet wet with radioactive water.

There seems to be a dearth of real data and research on the effect of radiation on people. And animals. Has anyone anywhere ever done animal experiments with the isotopes? I can't find anything on that.

 

[Bob S]

Here is a link to the TEPCO report on the three contractor workers who were working in a puddle of radioactive water in the basement of #3 turbine building, and the measured radioisotope concentration of that water. The measured dose rate at surface of puddle (≈ 15 cm deep?) was about 400 mSv/hr.

http://www.tepco.co.jp/en/press/corp-com/release/11032503-e.html

[added] Here is link to radioactivity analysis of water in the basement of #1 turbine.

https://www.physicsforums.com/showthread.php?t=480200&page=78

Here is further analysis of water in buildings #s 1, 2, 3 originally posted by Antoni in post #1428 of main thread.

https://www.physicsforums.com/attachment.php?attachmentid=33592&d=1301197946

Note the ">1000 mSv/hr" at top of column for water in building #2 . Based on ratio of cesium isotopes, may actually be higher.

Revised numbers for #2 reported by Pietkuip (post # 1576) in main thread.

http://www.tepco.co.jp/cc/press/betu11_j/images/110327o.pdf

(in English)

http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110327e15.pdf

Bob S

 

[robinson]

I can't find any research or even a published guide to radiation that describes the almost immediate (in an hour or so) damage to skin from that level of radioactivity. In fact, even much higher levels don't show damage to the skin in that time period.

What level of radioactivity is going to cause skin burns in that short of a time period?

 

[Bob S]

Here is a running record of the radiation levels in CAMS (continuous air monitor samplers) in units 1, 2, and 3 updated by Jorge Stolfi, in centi-sieverts/hr (rems/hr):

http://www.ic.unicamp.br/~stolfi/EXPORT/projects/fukushima/plots/cur/cams-un1.txt

http://www.ic.unicamp.br/~stolfi/EXPORT/projects/fukushima/plots/cur/cams-un2.txt

http://www.ic.unicamp.br/~stolfi/EXPORT/projects/fukushima/plots/cur/cams-un3.txt

I have updated again my plots of the reactor variables:
http://www.ic.unicamp.br/~stolfi/EXPORT/projects/fukushima/plots/cur/Main.html"

Also look at TEPCO news release

http://www.tepco.co.jp/en/press/corp-com/release/11040505-e.html

appendix 5 for April 4 update and reference 6 for running plots (totals) of on-site Dai-ichi air radioactivity analysis.

Bob S

 

[Bob S]

I was just looking at the Wiki article
http://en.wikipedia.org/wiki/Acute_radiation_syndrome#Exposure_levels
and wondering why no mention of skin problems from radiation, when they got their feet wet with radioactive water.

Here is a back-of-the-envelope calculation of the radiation level required to cause immediate skin burns.

The specific heat of tissue is about 4 joules per gram-degree C. So it would require about 80 joules/gram to raise the skin temperature 20 deg. C (like spilling boiling water on skin).

Because the definition of a Sievert is 1 joule of energy deposition per kilogram, we have

20 deg C temp rise = 80 joules per gram = 80,000 joules per kilogram = 80,000 Sieverts.

This sounds like a lot. For comparison, I know (from personal experience) that I could not feel 42 doses of 1.8 Sieverts (per session) of focused gamma radiation for prostate cancer treatment last year.

There seems to be a dearth of real data and research on the effect of radiation on people. And animals. Has anyone anywhere ever done animal experiments with the isotopes? I can't find anything on that.

There is a lot of information on acute radiation symptoms in humans gained from a variety of radiation accidents, reported in:

http://www.bordeninstitute.army.mil/published_volumes/nuclearwarfare/chapter2/chapter2.pdf

The extensive list of references could also include the results of controlled experiments on animals.

Bob S

 

[biscuitcheese]

Neutrons are charge neutral but have non-zero nuclear spin states so they can be detected by scattering in a magnetic material. Are there portable neutron detectors of this sort?

I guess you still need some form of interaction detector like a scintillating material + photodetector, but at least it can help eliminate background noise of other energetic particles being detected.

 

[robinson]

20 deg C temp rise = 80 joules per gram = 80,000 joules per kilogram = 80,000 Sieverts.

This sounds like a lot. For comparison, I know (from personal experience) that I could not feel 42 doses of 1.8 Sieverts (per session) of focused gamma radiation for prostate cancer treatment last year.

Thanks Bob. I am of the opinion this will be much like every other nuclear disaster. Not much real information, lots of nonsense, and the reality won't be known for 20 years, if ever.

Meanwhile, the latest report today says radiation levels are too high to measure. Which is probably what happened to the men who were burned. If they can't measure more than 1 sievert, they don't even know what the real levels were.

 

[Bob S]

Neutrons are charge neutral but have non-zero nuclear spin states so they can be detected by scattering in a magnetic material. Are there portable neutron detectors of this sort?

The best general purpose neutron detectors are the BF₃ or the He³ proportional counters, which work well on thermal neutrons, but not with pulsed neutron sources. A silver-foil-wrapped Geiger-Mueller tube works well in pulsed-neutron fields. This uses Ag¹⁰⁷ activation with a 2.3 minute half-life. Use in combination with a similar GM tube without silver foil to subtract ionizing radiation background. Don't know about any neutron detector using neutron scattering in a magnetic material. Would it work if the neutron source is isotropic (thermal neutrons)?

Bob S

 

[PietKuip]

Neutrons are charge neutral but have non-zero nuclear spin states so they can be detected by scattering in a magnetic material. Are there portable neutron detectors of this sort?

I guess you still need some form of interaction detector like a scintillating material + photodetector, but at least it can help eliminate background noise of other energetic particles being detected.

Magnetic materials can deflect neutrons, but that does not help if one does not know the direction where they are coming from.

Neutrons can be detected by gas counters with BF3 or helium-3, that capture neutrons. But yes, there is a problem with gamma backgrounds.

The easiest way to detect neutrons is to use a material with a large cross section for neutron activation and a suitable half-life. Indium foil, for example. Or a gold ring. Or a simple battery (it contains manganese oxide). After exposure, one can take it out, and measure the induced radioactivity in a place that is not contaminated. For qualitative estimates, a simple Geiger-Muller tube will do.

 

[Bob S]

...Meanwhile, the latest report today says radiation levels are too high to measure. Which is probably what happened to the men who were burned. If they can't measure more than 1 sievert, they don't even know what the real levels were.

NOT TRUE: "Meanwhile, the latest report today says radiation levels are too high to measure. " Where did you read this?

First, a properly designed radiation detector can handle 1 Sievert/second (100 rads/second) or 3600 Sieverts/hr. See Fig. 4 on page 50 in
http://beamdocs.fnal.gov/AD/DocDB/0010/001068/001/A tutorial on beam loss monitoring.pdf

Second, IF the men were carrying audible radiation detectors, then it should be a design that does not paralyze in high rad fields. I don't know.

Bob S

 

[biscuitcheese]

The best general purpose neutron detectors are the BF₃ or the He³ proportional counters, which work well on thermal neutrons, but not with pulsed neutron sources. A silver-foil-wrapped Geiger-Mueller tube works well in pulsed-neutron fields. This uses Ag¹⁰⁷ activation with a 2.3 minute half-life. Use in combination with a similar GM tube without silver foil to subtract ionizing radiation background. Don't know about any neutron detector using neutron scattering in a magnetic material. Would it work if the neutron source is isotropic (thermal neutrons)?

Bob S

I suppose ionization or neutron activation detectors are simplest and suitable for portability.

If the neutron source is directionally isotropic, can they use in tandem some form of neutron collimator to colliminate the beam first towards a magnetic material for diffraction towards the detector?

Magnetic materials can deflect neutrons, but that does not help if one does not know the direction where they are coming from.

Neutrons can be detected by gas counters with BF3 or helium-3, that capture neutrons. But yes, there is a problem with gamma backgrounds.

The easiest way to detect neutrons is to use a material with a large cross section for neutron activation and a suitable half-life. Indium foil, for example. Or a gold ring. Or a simple battery (it contains manganese oxide). After exposure, one can take it out, and measure the induced radioactivity in a place that is not contaminated. For qualitative estimates, a simple Geiger-Muller tube will do.

I suppose one can figure out the direction of the incident neutrons based on their scattering vector within a single magnetic domain of a crystalline magnetic material (assuming we have scintillator detector elements measuring in all directions e.g. philosophically similar to something like the Sudbury neutrino observatory)?

Or one can have the detector enveloped in a neutron capturing material like boron, except for an opening for the measurement aperture - which may also be fitted with a neutron collimator.

Alternatively one may also borrow the philosophy from high energy particle detectors and have some kind of 3D detection element contraption that can do 3D particle tracking?

Neutron activation is probably the simplest way for neutron detection but I guess they can suffer saturation so there will be dose limitations? Also is there temporal-resolution limitations in measurements? And also, isn't the neutron capture cross-sections of even high absorption materials like boron smaller than some of the magnetic scattering cross-sections (especially with the wide range of neutron energies)? Presumably then neutron activation detectors may be less sensitive than other detection approaches?

 

[clancy688]

I think this question fits here best...

Since there are several estimates of released radioactivity, I would like to try converting "released radioactivity of isotope x" into "released mass of isotope x". I think this would help in imagining, how much of the reactor's inventory is gone.
But how do I do this?

My approach would be the following, but I'm not sure if it's right, if it's an okay estimate or if it's total ********. So please give me some feedback. ^^;

I'd take the released activity (of the wanted isotope) in becquerel, multiply it with the half time (in seconds) and double the resulting number. This should give me the number of atoms released to begin with.
Afterwards, I'll just multiply with the isotope's atomic mass and hopefully I'll get the released mass in kg.

I tried this with the IRSN release estimate for Cäsium-137 (10.000 TBq between 12th and 22th of March) and got ~4kg of Cäsium-137.

I'm no reactor expert, is a release of 4kg Cäsium-137 in such an accident (damaged fuel rods, continuous containment venting, damaged fuel rods in SFPs) to much or to less?

How much Cäsium-137 is there in Fukushima overall in all damaged reactors? (1-4)

 

[IowaNewbie]

Lurker now just popping in for clarification:

NOT TRUE: "Meanwhile, the latest report today says radiation levels are too high to measure. " Where did you read this?

It was from a news article in today's NHK web site: http://www3.nhk.or.jp/daily/english/05_38.html"

 

[Astronuc]

Lurker now just popping in for clarification:

It was from a news article in today's NHK web site: http://www3.nhk.or.jp/daily/english/05_38.html"

That probably survey monitors. They could be used if shielded, but then they'd have to be recalibrated. Basically those monitors are designed for 'normal' levels, not for high rad fields as is the case. Even in an emergency or 'accident', no one at TEPCO expected/anticipated in dealing with the current levels of radiation.

 

[Bob S]

I suppose ionization or neutron activation detectors are simplest and suitable for portability.

If the neutron source is directionally isotropic, can they use in tandem some form of neutron collimator to colliminate the beam first towards a magnetic material for diffraction towards the detector?

I suppose one can figure out the direction of the incident neutrons based on their scattering vector within a single magnetic domain of a crystalline magnetic material (assuming we have scintillator detector elements measuring in all directions e.g. philosophically similar to something like the Sudbury neutrino observatory)?

Or one can have the detector enveloped in a neutron capturing material like boron, except for an opening for the measurement aperture - which may also be fitted with a neutron collimator.

Alternatively one may also borrow the philosophy from high energy particle detectors and have some kind of 3D detection element contraption that can do 3D particle tracking?

Neutron activation is probably the simplest way for neutron detection but I guess they can suffer saturation so there will be dose limitations? Also is there temporal-resolution limitations in measurements? And also, isn't the neutron capture cross-sections of even high absorption materials like boron smaller than some of the magnetic scattering cross-sections (especially with the wide range of neutron energies)? Presumably then neutron activation detectors may be less sensitive than other detection approaches?

The "Albatross" pulsed-neutron detector was developed at NAL (Fermilab) in 1970 using the foil activation technique suggested by Alan Smith at UCRL in 1962

http://escholarship.org/uc/item/5nm1k5qw;jsessionid=61B611B89D49E6ABFF8A2A8B910DBC40#page-1

http://cdsweb.cern.ch/record/864514/files/p1035.pdf

See also CERN Courier Vol 10 # 12 (1970) page 392

The objective was to develop a neutron detection instrument for use in personnel areas adjacent to accelerator beam enclosures where pulsed particle beams were present. Foil activation decay lifetimes between about 15 to 200 seconds were considered to be optimum. Both indium and silver foils were acceptable. Gold has a high activation cross section, but the decay lifetime is much too long. The objective was to protect personnel from neutron radiation, and not to find the neutron source. In order to thermalize high energy neutrons, Two small Geiger tubes, one wrapped with silver foil, were surrounded by about a 25-cm diameter (8 Kg) pseudosphere of polyethlene. This size was selected based on optimizing the response to the energy spectrum of accelerator produced neutrons. This instrument required AC power, and was not very portable.

For high radiation levels, an instrument using a tissue equivalent ionization chamber was developed.

Several newer versions of the Albatross have been developed at other laboratories.

Bob S

 

[biscuitcheese]

Isnt saturation a problem with these [neutron activation] types of detectors? I guess they work well for bursts of neutrons but what about continuous beams?

Also, what mechanisms are relied upon in these detectors to cancel the noise background of gamma rays that the GM tube may also pick up?

I suppose the source of neutrons in the case of accelerators will be obvious and so the only concern is measurement of dose levels. In the case of active disaster mitigation or failure analysis where one may need to isolate the source of neutrons presumably to look for cracks in a reactor building or even find the source of ongoing fission, one may need a detector that can do continuous measurements. So how is this done?

 

[mantrogo]

Hi Clancy688 - more or less the same question occurred to me - this is how I did it:

the Bq number gives you the activity of a quantity raqdioactive material where one nucleus decays per second and is tied into atomic mass and half life by following equation:

Bq = (m / ma) * Na * (ln(2) / t1/2)

m=mass in grams, ma = atomic mass, Na = avogadro constant, t1/2=half life in seconds

if you had Cs137 at 10,000TBq and half life is 30.17 yrs

then m= Bq / (Na * (ln(2) / t1/2)) * ma => m = 10E15 / (6.02E23 * (ln(2) / 9.51E8))

m = 0.166 grams

 

[PietKuip]

Neutron activation is probably the simplest way for neutron detection but I guess they can suffer saturation so there will be dose limitations?

No, this does not need to saturate. It would be very difficult to transmute any significant fraction of the stable isotope. Of course the activated sample might saturate the GM-counter, but that can be solved by measuring the actvity of a part of the sample, or increasing the distance.

Also is there temporal-resolution limitations in measurements?

Yes. It depends on one's choice of the stable isotope. In the world's first pile which had 0.5 watt thermal power, Fermi used rhodium foil, of which the induced radioactivity has a half-life of 44 seconds. Activated indium has a halflife of an hour, manganese three hours, gold three days.

And also, isn't the neutron capture cross-sections of even high absorption materials like boron smaller than some of the magnetic scattering cross-sections (especially with the wide range of neutron energies)? Presumably then neutron activation detectors may be less sensitive than other detection approaches?

In our course lab, students measure the flux of our 3 mCi Ra/Be source, which is about 80 neutrons per cm2 per second. Higher fluxes are easier.

 

[PietKuip]

Isnt saturation a problem with these [neutron activation] types of detectors? I guess they work well for bursts of neutrons but what about continuous beams?

Also, what mechanisms are relied upon in these detectors to cancel the noise background of gamma rays that the GM tube may also pick up?

One can take the activated sample to a space with low gamma background.

I suppose the source of neutrons in the case of accelerators will be obvious and so the only concern is measurement of dose levels. In the case of active disaster mitigation or failure analysis where one may need to isolate the source of neutrons presumably to look for cracks in a reactor building or even find the source of ongoing fission, one may need a detector that can do continuous measurements. So how is this done?

One can do measurements integrated over time. The maximum integration time is given by the half-life of the induced radioactivity of the sample.

 

[clancy688]

then m= Bq / (Na * (ln(2) / t1/2)) * ma => m = 10E15 / (6.02E23 * (ln(2) / 9.51E8))

m = 0.166 grams

Thx. But I tried it out with your formula and got 3120g, which is surprisingly close to my first estimate of 4300g.

I think you made a little mistakes. You forgot the atomic mass ma in your final equation. And even if I calculate your term, I don't get your 0.166 grams as solution... ^^;

(10E15 / (6.022E23 * (ln(2) / 9.51E8))) * 136.907 ~ 3120 grams

 

[mantrogo]

*roll*eyes" - what`s that saying about haste and speed again..?

thankyou, thankyou, my apologies - should have looked at what I was writing!

 

[biscuitcheese]

No, this does not need to saturate. It would be very difficult to transmute any significant fraction of the stable isotope. Of course the activated sample might saturate the GM-counter, but that can be solved by measuring the actvity of a part of the sample, or increasing the distance.

I guess I meant by saturation, the consumption of the neutron capturing elements. Yeah, I guess depending on the choice of detector (GM tube or solidstate photodetector etc), that can be saturated too, so whichever is the bottleneck.

Yes. It depends on one's choice of the stable isotope. In the world's first pile which had 0.5 watt thermal power, Fermi used rhodium foil, of which the induced radioactivity has a half-life of 44 seconds. Activated indium has a halflife of an hour, manganese three hours, gold three days.

But is one advantage, of say a measurement method using scintillating material with a tandem photodetector, that being the temporal resolution of measurement can be much higher?

Also, for the case of activation based detection, would one not need multiple times the half-life to ensure higher precision of measurement, if the neutron flux is unknown at the time of measurement and that it is fluctuating (because the temporally measured signals are combination of many elements at different stages of decay and that the neutron flux was not constant during measurement)? If so, would that not further degrade temporal resolution?

In our course lab, students measure the flux of our 3 mCi Ra/Be source, which is about 80 neutrons per cm2 per second. Higher fluxes are easier.

I am guessing that it is easier to measure with higher flux because there are more chances for neutron capture by the foil. But that would mean that the detector is inherently not very sensitive because most of the neutrons escape capture and thus their detection.

What would be the solution for measuring neutrons with high sensitivity? What method has the highest neutron interaction cross-section (capture, scattering, etc)?

One can take the activated sample to a space with low gamma background.

Yea, but I suppose this method fails to provide real time information that may be vital for certain situations like real-time neutron detection in a reactor disaster such as the one in japan.

So essentially the common neutron detectors will fail to discriminate between gamma rays and neutrons?

One can do measurements integrated over time. The maximum integration time is given by the half-life of the induced radioactivity of the sample.

I suppose this method can work to reveal quasi-real-time measurements until the activation foil is consumed or saturates to the point of lower effectiveness in capturing neutrons -even for the same flux for example)?

 

[Bob S]

Isnt saturation a problem with these [neutron activation] types of detectors? I guess they work well for bursts of neutrons but what about continuous beams?

Analysis of silver foil activation for a 10⁷ n/cm² neutron pulse and for 10⁷ n/cm²-hr continuous neutron flux.

Consider a short pulse of 10⁹ thermal neutrons per cm², which has a rem dose equivalent of ≈ 1 rem (= 0.01 Sievert). See

See table G-17 in http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10098

What would be the activation in 1 gram of silver foil? The thermal neutron activation
cross section is 45 barns. See

http://www.nndc.bnl.gov/sigma/getPlot.jsp?evalid=4356&mf=3&mt=1&nsub=10

$$N = ({10^9 n/cm^2}) (45 x 10^-^2^4 cm^2)(6x10^2^3/107)=2.5x10^8 atoms$$

The decay rate is (with a 142 second half life)

$$Bq=(2.5x10^8) (ln(2)/142s) = 1.2x10^6$$ atoms per second.

For continuous activation at 10⁹ neutrons per cm²-hr, the decay rate equals the activation rate.

Bq=(2.5x10⁸)/3600) = 70,000 decays per second.

The Geiger tube is probably ≈25% efficient in detecting Ag¹⁰⁸ decays.

More likely thermal neutron pulses and fluxes are 10⁶/cm² and 10⁶/cm²-hr, representing 1 mrem and 1 mrem/hr (0.01 mSv and 0.01 mSv/hr).

Also, what mechanisms are relied upon in these detectors to cancel the noise background of gamma rays that the GM tube may also pick up?

Use two GM tubes, only one with silver foil, one without, and measure the difference.

I suppose the source of neutrons in the case of accelerators will be obvious and so the only concern is measurement of dose levels. In the case of active disaster mitigation or failure analysis where one may need to isolate the source of neutrons presumably to look for cracks in a reactor building or even find the source of ongoing fission, one may need a detector that can do continuous measurements. So how is this done?

Use variant of above detector. If neutrons are all thermal, then no moderator is required. BF₃ or He³ detectors are good choices. For high fluxes, maybe an ion-chamber enhanced with silver foil will work.

Bob S

 

[biscuitcheese]

Use two GM tubes, only one with silver foil, one without, and measure the difference.

Use variant of above detector. If neutrons are all thermal, then no moderator is required. BF₃ or He³ detectors are good choices. For high fluxes, maybe an ion-chamber enhanced with silver foil will work.

Bob S

I know some neutron detectors use two GM tubes with one having a neutron capturing foil and rely on the differential measurements as a form of neutron detection. I see they can be effective as merely a detector of neutrons - since the neutron activated foil will leave one GM tube with higher activity reading than the other (but even so not for continuous beams of unknown, possibly high, neutron/gamma fluxes).

However I am wondering as to their true effectiveness in a fluctuating high gamma and neutron radiation background since the gamma may saturate the signals of both GM tubes rendering differential readings ineffective? Also, unless they are in some kind of high density dual GM tube array, would such a detector (if they only have two GM tubes) not be very directionally dependent in their detection?

I thought most of the differential GM tube detectors with the activation foil tend to have a UHMWPE outer shell as neutron moderators to thermalize the neutrons first to improve effectiveness of activating the foil?

 

[Bob S]

I know some neutron detectors use two GM tubes with one having a neutron capturing foil and rely on the differential measurements as a form of neutron detection. I see they can be effective as merely a detector of neutrons - since the neutron activated foil will leave one GM tube with higher activity reading than the other (but even so not for continuous beams of unknown, possibly high, neutron/gamma fluxes).

Also works in continuous neutron fluxes.

However I am wondering as to their true effectiveness in a fluctuating high gamma and neutron radiation background since the gamma may saturate the signals of both GM tubes rendering differential readings ineffective? Also, unless they are in some kind of high density dual GM tube array, would such a detector (if they only have two GM tubes) not be very directionally dependent in their detection?

Not directionally dependent for neutrons and gammas from beta decay. In the circuit, the Geiger tube pulses are each converted to analog signals with a time constant of several seconds, and one signal is subtracted form the other. The Geiger tubes are very small, and work in high fluxes of neutrons and/or gammas.

I thought most of the differential GM tube detectors with the activation foil tend to have a UHMWPE outer shell as neutron moderators to thermalize the neutrons first to improve effectiveness of activating the foil?

See attached picture of "albatross" dual Geiger tube pulsed and continuous neutron flux detector (from 12/1970 CERN Courier) with 12.5 cm diameter polyethylene moderator.

Bob S

 

[yoshibello]

Hi, Question
Could you tell me in layman terms what I would be looking for if I were testing Alaska seafood/produce. What would be the parameters for a hand held radiation detector? I want to purchase a detector, but want to get the right kind for the application. Will it display all types of radiation? Your help is much appreciated. Thank you. Chris

 

[PietKuip]

Hi, Question
Could you tell me in layman terms what I would be looking for if I were testing Alaska seafood/produce. What would be the parameters for a hand held radiation detector? I want to purchase a detector, but want to get the right kind for the application. Will it display all types of radiation? Your help is much appreciated. Thank you. Chris

The limits for food are so low that a handheld detector does not really show much. It needs a lead chamber (to reduce background from floor and walls) and longer counting times to detect anything.

 

[biscuitcheese]

Also works in continuous neutron fluxes.Not directionally dependent for neutrons and gammas from beta decay. In the circuit, the Geiger tube pulses are each converted to analog signals with a time constant of several seconds, and one signal is subtracted form the other. The Geiger tubes are very small, and work in high fluxes of neutrons and/or gammas.
See attached picture of "albatross" dual Geiger tube pulsed and continuous neutron flux detector (from 12/1970 CERN Courier) with 12.5 cm diameter polyethylene moderator.

Bob S

I know it can work in a continuous neutron flux. I was getting at the idea that there should be a dose limitation at which point the efficiency of neutron capture by the film will decrease? Alternatively the GM tube reaches a saturation in signals. In both cases, what sort of dose are we talking about in a common neutron detector such as the dual GM tube construction ones you are talking about??

I mean by directionally dependent, as in if the GM detectors are cylindrical and axially parallel, and there is a highly collimated narrow beam of neutrons or gamma photons, then it is possible that say the gamma beam can irradiate on the foil covered GM tube leading to a reading while the non-foil GM tube shows no reading, leading to a false positive. This is purely hypothetical of course, and can easily be overcome by reducing GM tube size, and by using an array construction. In any case, this detection method is not error prone and works best for detection for neutron pulses with low gamma background for which one can definitively determine the difference in detection between the two GM tubes?

 

[yoshibello]

PietKulp--thanks-I will try to find a detector with these parameters. I want to get as many chefs testing as possible for a safe source of fish, thus putting pressure on the FDA, NOAA and the seafood orgs to come clean and get the processors to test each tender ship. Questions; 1)what would be the normal counting time? 2)Do all detectors test for all 3 radiation types coming from Fukushima? Thanks again, Chris

 

[PietKuip]

PietKulp--thanks-I will try to find a detector with these parameters. I want to get as many chefs testing as possible for a safe source of fish, thus putting pressure on the FDA, NOAA and the seafood orgs to come clean and get the processors to test each tender ship. Questions; 1)what would be the normal counting time? 2)Do all detectors test for all 3 radiation types coming from Fukushima? Thanks again, Chris

I do not think that this is something chefs should do. The FDA does not need pressure, they will do it. American authorities will screen imported food from Japan. It is horribly expensive to buy the sensitive equipment that they have.

Counting times depend on how sensitive the detector is. Geiger-Muller tubes are not sensitive. One needs NaI (sodium iodide) scintillator crystals, the larger the better.

Such detectors are for gamma radiation, which will give a good idea of what radioactive isotopes are present, if you can measure the spectrum.

So this is expensive equipment. And still, I would not expect to it to be able to detect radiactivity in seafood. If you are worried, you can eat pre-March canned or frozen fish for a while.

 

[biscuitcheese]

PietKulp--thanks-I will try to find a detector with these parameters. I want to get as many chefs testing as possible for a safe source of fish, thus putting pressure on the FDA, NOAA and the seafood orgs to come clean and get the processors to test each tender ship. Questions; 1)what would be the normal counting time? 2)Do all detectors test for all 3 radiation types coming from Fukushima? Thanks again, Chris

Most handheld detectors rely on ionization to detect radiation, either gas ionization detectors (such as geiger muller tube detectors) or variants of solid state ionization detectors.

Depending on your choice of the detector it may not detect all three radiation types, e.g. some solid state diode detectors are primarily only sensitive to gamma photons. Furthermore, since the energetic particles released from radioactive decay or fission have different penetration depths through matter, such as through air, you have to bring the detector element close enough to the source in order to detect them (in particular, alpha and beta particles). Most detectors are designed and used for gamma detection. Alpha and beta detection will require special detectors used in a specific fashion, otherwise it will be ineffective.

In other words, if you are testing fish, your handheld detectors are probably good for detecting gamma rays, since they have long penetration depths (certainly enough to penetrate the fish). You may also be able to pick up beta decay with some detectors if you are close enough. But for alpha decay, you'd have to be _really_ close to the source which means if the fish ingested the source, and you are only scanning the outside of the fish from a good distance like most people are shown to be doing on TV, you wouldn't be able to pick it up.

 

[yoshibello]

PietKiup and Biscuitcheese--Thanks again. More info, more questions! Is the radiation from Fukushima gama photons? I would be testing directly on the fish. So if I'm interpreting this correctly, a solid state ionization detector is better than gas ionization.
Would the following be sufficient sensitivity:
1000 cpm/mR/hr or 108 pulses referenced to Cobalt-60 radiation of 1 µSv/h ambient
Alpha - from 4.0 MeV
Beta - from 0.2 MeV
Gamma - from 0.01 MeV
for radiation detection on a fish up close and personal? This unit is a halogen filled geiger muller?

 

[biscuitcheese]

I am a believer that in our free country people should not be denied the right to know. So if you are interested in knowing if your food/ingedients are 'radioactive' you should be allowed to have a way of checking it yourself also since food safety authorities ultimately only check by sampling.

However in general I think the health and safety considerations of food is ultimately best left to the federal health and safety authorities in your country.

Short of comparing to academic/government/commercial research labs, sometimes the health/safety authorities will even have better equipment and better knowledge on experimentation techniques than the average civilian. This can ensure you will not get a false negative or false positive results which can have conceivably bad consequences.

For example, I'd imagine a very due dilligent health and safety regulator can sample fish properly from a large batch and not only use proximity radiation detectors to detect for high levels of radioactive decay that will be evidence for contamination, but also process some food samples for mass spectrometry to detect trace concentrations of radioisotopes if they exist, such as alpha emitters that can otherwise escape detection, etc. No average civilian has mass spectrometers or photospectrometers (the best they can do is spend a few thousand on a radiation detector).

There are too many commercially available types of detectors to draw a generic answer. Different detectors will have different characteristics, so you need to choose the one best for your application. The detector you have is a conventional GM counter.

It will depend on the extent and nature of the actual contamination from this fukushima incident for which nobody really knows yet. However there are many exotic radioisotopes produced from fission, which your fish may or may not have ingested based on radioisotpes that may or may not have been released into the environment. Again nobody knows. Assuming the worst case (worst case being the fish is contaminated with all sorts of radioisotopes and still managed to survive capture into the food chain) then all three emitters need to be considered.