Fukushima 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 [Broken]

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

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Bob S

This chart from

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

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

Bob S

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robinson

Fascinating. I was just looking at the Wiki article
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

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

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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 [Broken]

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

http://www.ic.unicamp.br/~stolfi/EXPORT/projects/fukushima/plots/cur/cams-un3.txt [Broken]
I have updated again my plots of the reactor variables:
http://www.ic.unicamp.br/~stolfi/EXPORT/projects/fukushima/plots/cur/Main.html" [Broken]
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

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Bob S

I was just looking at the Wiki article
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.

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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, isnt 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?

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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" [Broken]

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Astronuc

Staff Emeritus
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" [Broken]
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.

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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, isnt 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

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 continous 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, isnt 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 continous 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)?

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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

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 continous 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

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