7 halflives

[Wlrun3]

I was asked why a large quantity of a relatively long half life nuclide would be detectable after 7 halflives and a minor quantity, not.
I was dissatisfied with my answer, mumbling something about limits of detection.
Can anyone help.

[Marlin]

I was asked why a large quantity of a relatively long half life nuclide would be detectable after 7 halflives and a minor quantity, not.
I was dissatisfied with my answer, mumbling something about limits of detection.
Can anyone help.

I think you are on track. Part of the problem may be an old Navy idiom that activity is gone after 7 half lives. 0.78% of a large quantity may still be detectable or even still a large quantity if you are talking in cosmological terms. 

[Wlrun3]

I finally located the origin of 100 dpm per 100 cm squared in NRC Inspection and Enforcement Circular 81-07.

"For smears of a 100cm2 area (a de facto industry standard), the corresponding detection capability with a thin window detector and a fixed sample geometry is on the order of 1000 dpm (i.e. , 1000 dpm/100 cm2). Therefore, taking into consideration the practicality of conducting surface contamination surveys; contamination control limits should not be set below 5000 dpm/100 cm2 total and 1000 dpm/ 100 cm2 removable. The ability to detect minute, discrete particle contamination depends on the activity level, background, instrument time constant, and survey scan speed."

This, for me, accounts for the CA and all personnel and material release limits. 

Will I find that, just as obscure, the origins of "non-detectable after 7 half lives" rests in a moment of simple necessity, rarely questioned.

"Every science starts as philosophy and ends as an art."
Will Durant, The Story of Civilization

[RDTroja]

Seven half-lives is an over simplification. It is just the integer threshold that gets you to under 1% of your original value... 6 half lives = 1/64, 7 = 1/128. Everybody knows that once you have 1% or less of something it is gone... or not.

Less than 1% of 1 mR is essentially gone in a power plant environment, but less than 1% of 10 Curies is not. 7 Half-lives of a chemistry sample is probably sufficient to call it innocuous (depending on the system.) Seven half-lives of a spent fuel bundle, not so much.

It is not limits of detection that are important. It is how much you started with in the first place.

[spentfuel]

I finally located the origin of 100 dpm per 100 cm squared in NRC Inspection and Enforcement Circular 81-07.

"For smears of a 100cm2 area (a de facto industry standard), the corresponding detection capability with a thin window detector and a fixed sample geometry is on the order of 1000 dpm (i.e. , 1000 dpm/100 cm2). Therefore, taking into consideration the practicality of conducting surface contamination surveys; contamination control limits should not be set below 5000 dpm/100 cm2 total and 1000 dpm/ 100 cm2 removable. The ability to detect minute, discrete particle contamination depends on the activity level, background, instrument time constant, and survey scan speed."

This, for me, accounts for the CA and all personnel and material release limits. 

Will I find that, just as obscure, the origins of "non-detectable after 7 half lives" rests in a moment of simple necessity, rarely questioned.

A comment on that....

From an old NRC reg guide

"A standardized method for smear testing of a relatively uniform area should be used to aid in comparing contamination at different times and places. A dry smear taken from an area of about 100 cm2 is acceptable to indicate levels of removable contamination."

and

"Note on Units: The above units of µci/cm2 have been used in this table since they are consistent with units adopted as national standards in several other nations and the IAEA (see Reference 6); the units of µCi and cm are already used to express concentration in 10 CFR Part 20, and they are readily convertible to SI units by the well-known relation; I µCi = 3.7x 10E04 dis/sec = 3.7 x 10E04 Becquere.s (Bq). They may also be easily converted to other frequently used units of radiation protection practice, i.e., disintegrations/minute per 100 cm2 2.22 x 10* x (activity expressed in µ.Ci/cm )."

As near as I recall it started back in the 50's but the standard unit was uci/cm2 and was applied mainly to laboratories and medical facilities.  When commercial nuclear power came to be it was not considered very practical to smear a cm2 and equally not practical to smear a m2 so it was agreed that after consideration of the type of contaminates being smeared and the potential for smear efficiency based on particulate loading etc that an area roughly 4" X 4" was a representative area.

Also 7 half lives is considered a "rule of thumb"  10 half lives is considered absolute

sf

[Wlrun3]

Do you know of any documented reference to the thumb rule for 7 half lives.

Where and when was the first smear documented in dpm/100 cm squared taken.

[spentfuel]

Rule of thumb comes from the 1970 edition of the Rad health Handbook page 205 which sites Los Alamos 3rd edition and general dynamics 1963 rad monitoring handbooks.

The 10 half lives comes from agreement state regulations for "Decay in storage" as an authorized method of waste disposal.

As for the first 100cm2 documentation my guess would be from valacetos atomic laboratories because the BWR's there had the first AEC license

hope that helps

sf

[GLW]

Sommers 1975

[bsdnuke]

Hi all,

The seven half lives or ten half lives are just really small numbers.  Multiply by a really big number and you still have radioactivity detectable.  In reality always there though there was a health physics article many years ago that talked about the probability of zero activity calculation.  I will look for it and get the abstract.

The 5000 dpm per hundred cm2 was an approximation that came out of weapons testing era and was what you could reliably detect with a pancake detector then as is now.  Of course now we have better science and can correlate the number to a dose like in MARSSIM.  But back then it was all about being able to release real property contaminated by above ground weapons testing.

I will get back with that article.

bsd

[bsdnuke]

Ok, the article is :"A Discussion of the Approximate Nature of The Exponential Decay Law as Applied to Radioactive Nuclides and Gamma Ray Absorption" by Herbert L. Jackson, Health Physics 11, 179-183.  This is where we got the idea of 7 half lives from.

If you are a member of HPS you can download this article from the website, http://www.hps.org

bsd

[Wlrun3]

There, finally, the origin...the questioner mentioned at this threads beginning was a non - navy sro now mba holding GE refuel floor leader and a municipal judge in his home plant area, oswego.
I admire and respect him.
Yes, hps member since 89.
Thankyou .

[Fermi2]

There, finally, the origin...the questioner mentioned at this threads beginning was a non - navy sro now mba holding GE refuel floor leader and a municipal judge in his home plant area, oswego.
I admire and respect him.
Yes, hps member since 89.
Thankyou .

Huh?

[Wlrun3]

He became an sro at nine mile, was not in the navy, attained an mba, now works for GE as one of their managers at bwr refueling outages, mainly on the refuel floor.
He serves as a judge in his community near oswego.
He considers me to be knowledgable in the field of Radiation Protection and asked me how a very large quantity of radioactivity could be considered, as the often mentioned thumb rule states, non detectable after 7 halflives.
As you have noticed, I cite references.
The above reply to my post provided that reference.
Thankyou for requiring clarity in my posts.

[Fermi2]

Oh The guy who asked you about the 7 half lives!

[cypher89]

I would imagine it depends on the isotope and which decay chain it is in.  Since a half life just is a measure of the absolute quantity of a material after a certain time. So something that emits high energy particles is easier to detect then something with low energy particles.  Or another way it takes less quantity of of high energy source to be detected then an low energy source.

[HAIRDUDE]

ALSO .... One must be sure to take into account that the isotope of greatest concern at the front end of the decay sequence will not necessarily be the isotope of greatest concern as time passes. Much as one might order a Combo Platter at a Chinese Restaraunt, comprised not only of shrimp, but beef, pork, chicken and crab meats as well (Oh Happy Family ... My fav) There can exist a great number of isotopes in combination of varrying decay rates. One should never assume that the "shrimp" having a relatively short half life will be of primary concern after it has decayed down and the other isotopes, metaphorically the beef, chicken etc., come into play. The ratios of short and long-lived isotopes must be taken into account in such instances. While the shrimp may have exhausted seven half leves, that beef may have only gone through two while the crab has yet to complete one.

I hope I haven't muddied the waters with this aside ..... All I know now is that I'm FRIGGEN HUNGRY for some reason ....