Fukushima radiation detection and measurement

Bob S

(This a new thread as proposed by Borek post #2349)
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/p...rs/bf3info.htm

http://www.gepower.com/prod_serv/pro...e3_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/i...equivalent.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/resea...otopeSheet.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_r...xposure_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...1032503-e.html

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

http://www.physicsforums.com/showthr...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.

http://www.physicsforums.com/attachm...2&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/betu...es/110327o.pdf

(in English)

http://www.tepco.co.jp/en/press/corp.../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/EXP...r/cams-un1.txt

http://www.ic.unicamp.br/~stolfi/EXP...r/cams-un2.txt

http://www.ic.unicamp.br/~stolfi/EXP...r/cams-un3.txt
Also look at TEPCO news release

http://www.tepco.co.jp/en/press/corp...1040505-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

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 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/...2/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

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

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

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

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/00...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

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?

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:

It was from a news article in today's NHK web site: Plant radiation monitor says levels immeasurable

Astronuc

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

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/5nm1...10DBC40#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