Liquid scintillation solution for measuring .sup.222 RN in charcoal

A solution which increases the efficiency of radon extraction from either dry or moist charcoal samples is disclosed. The addition of low molecular weight organic solvents such as methanol or ethanol to liquid scintillation counting solution increases radon extraction from 20-500% over known counting solutions. The increased efficiency appears to occur from increased penetration of the charcoal by the low molecular weight solvent which dissolves the radon gas and the subsequent diffusion of the low molecular weight solvent out of the charcoal and into the liquid scintillation counting solution.

BACKGROUND OF THE INVENTION 
This invention relates to radon extraction solvents. Typically, low 
molecular weight (LMW) organic solvents may be added to conventional 
liquid scintillation counting (LSC) solutions which have poor or 
suboptimal abilities to extract radon gas (.sup.222 Rn) from charcoal 
thereby producing LSC solutions which have optimal abilities to extract 
.sup.222 Rn from charcoal for measurement of the radiation from .sup.222 
Rn and its radioactive daughter products. The invention also relates to 
the ability of these same solvents when added directly to charcoal to 
efficiently displace radon from the charcoal and into the gas and/or 
solvent phases which may be in close proximity to the charcoal. Many 
methods have been described for the measurement of .sup.222 Rn in air. 
Several methods rely upon the ability of activated charcoal to adsorb and 
retain .sup.222 Rn gas from the air. Subsequently, the radioactivity in 
the charcoal may be measured by direct gamma counting of the charcoal or 
alternatively by liberating the .sup.222 Rn gas from the charcoal within a 
closed system for radioactive measurement. With the latter method, for 
high measurement sensitivity and accuracy it is important that .sup.222 Rn 
liberation is efficient and reproducible. Only one previous study 
(Pritchard and Marien, Anal. Chem. 1983, 55, 155-157) has reported a 
method for liberation of .sup.222 Rn from charcoal into liquid 
scintillation solutions for radioactive counting. Using gamma counting of 
the original charcoal and of the separate solvent extract, the above study 
showed that after submerging granular charcoal in pure toluene for at 
least two hours and then removing the toluene, almost all of the radon 
could be accounted for in the toluene. However, quantitative solvent 
extraction or even high efficiency radioactive counting of radon was not 
demonstrated when charcoal remained mixed with the toluene (or any other 
solvents) or when the toluene contained dissolved fluors necessary for 
liquid scintillation counting. Pritchard and Marien reported that they 
obtained reproducible but not necessarily quantitative extraction of 
.sup.222 Rn comparing the (cpm) counts per minute data among multiple LSC 
vials having "2 g carbon residing on the bottom of the vial" containing a 
toluene liquid scintillation solution with unspecified fluors. It was 
separately demonstrated by Perlman that warming a charcoal sample which is 
either submerged or placed directly above an aromatic solvent based liquid 
scintillator (containing toluene or xylene for example), accelerates the 
displacement of .sup.222 Rn from the charcoal into the liquid 
scintillator. 
The above prior art methods using toluene or other monocyclic aromatic 
solvents based liquid scintillators for extracting and counting radon in 
free granular charcoal suffer from one or more disadvantages including 
sub-optimal radon extraction efficiency in counting dry and especially 
moist (water-containing) charcoal samples, toxicity of the toluene solvent 
and vapor, and inconvenience and costs involved with incubation, or in the 
handling, loading, and disposal of the toxid liquid scintillator and the 
charcoal sample. 
One object of the present invention is to provide a liquid scintillator 
which exhibits improved radon extraction efficiency in counting both dry 
and moist charcoal samples. 
Another object of the invention is to provide a liquid scintillator which 
is non-toxic and is therefore safe and convenient to use, handle and to 
later recycle or discard. 
A further object of this invention is to provide inexpensive solvents which 
are compatible with liquid scintillator solutions and which can be applied 
directly to charcoal to efficiently and rapidly displace the radon 
contained within the charcoal. 
SUMMARY OF THE INVENTION 
The invention involves the utilization of one or more radon extraction 
solvents, typically low molecular weight (LMW) solvents such as ethanol to 
release radon from charcoal. The previously unrecognized ability of 
certain solvents to perform this task permits formulation of new liquid 
scintillation (LS) solvent mixtures (commonly termed "cocktails") which 
show improved efficiency and speed of extraction of .sup.222 Rn from 
charcoal. Typically, the radon extraction solvents have molecular weights 
between 30 and 100 daltons. For example two of these, methanol and ethanol 
have molecular weights of 32 and 46 daltons respectively. 
Conventional LS cocktails typically consist of an aromatic LS counting 
solvent (e.g., benzene, toluene, xylene, pseudocumene, didodecyl-benzene 
or other alkyl-benzenes, or multiaromatic-ringed solvents) and a dissolved 
fluor (e.g., PPO-POPOP). The fluor molecules emit light, i.e., fluoresce 
when excited through electron transfer in the aromatic solvent, initiated 
by an alpha or beta particle emitted during a radioactive decay event. The 
fluorescence events are typically detected in an LSC instrument by 
coincidence-coupled phototubes which electronically convert the 
scintillant light flashes into counts per minute. LS cocktails may also 
contain various detergents, solubilizers and emulsifiers which are 
carefully chosen to sustain optical clarity and miscibility when a variety 
of liquid samples are added to the cocktail. Pritchard and Marien (1983) 
discovered that radon could be extracted from charcoal by toluene and 
thereafter counted in a conventional toluene solvent-based LS cocktail. 
Incubation of radon-bearing activated charcoal in xylene or toluene-based 
cocktails results in extraction-equilibration and radon-daughter ingrowth 
within typically 6-12 hours depending upon the quantity and size of 
charcoal granules. This method gives reproducible counting results, but 
the efficiency of radon extraction is not optimized and the interference 
of moisture in the charcoal with efficient radon extraction is not 
recognized. It was believed that xylene and toluene which show high 
chemical partition coefficients for radon (.about.50-fold greater than 
water's partition of radon) would be among the best solvents for 
extracting radon from charcoal. Many variations on the Pritchard method 
have been developed (Perlman, 1986) including a rapid extraction procedure 
carried out by incubating the charcoal granules in a xylene-based LSC 
cocktail for 5 minutes at 80.degree.-100.degree. C. However, this solvent 
immersion-heat method and the method of heating charcoal positioned in the 
gas phase above the LSC cocktail do not substantially improve the final 
efficiency of radon extraction compared to that achieved more slowly at 
room temperature. 
Recently diphenyl derivative solvent and long chain alkyl benzene solvent 
based LS cocktails have been tested for their abilities to extract radon. 
Unfortunately, some of these cocktails whose solvents are much less 
volatile and toxic than toluene or xylene, showed as little as 20% the 
radon extraction efficiencies shown by the prior art toluene and 
xylene-based cocktails. Experiments were performed to understand and solve 
the problem of poor radon extraction efficiency. It was found that 
although such low toxicity or essentially non-toxic high molecular weight 
aromatic solvents had high partition coefficients for radon, these larger 
aromatic molecules poorly infiltrated the small charcoal channels and 
spaces holding the radon atoms. It is now believed that high efficiency 
radon extraction from charcoal demands a solvent which can first penetrate 
into the very small adsorption channels within the charcoal, then dissolve 
the radon and finally diffuse out of the charcoal to liberate the radon. 
Empirically, it has been discovered that some LMW organic solvents are 
very efficient in performing this extraction even when diluted into 
diphenyl-derivative solvent-based and long chain alkyl benzene-based LSC 
cocktails which otherwise extract radon from charcoal very poorly. Several 
of these LMW organic solvents which are polar, are soluble or miscible in 
both water and in the LSC cocktail aromatic solvents. This solvent 
cross-solubility in water and LSC cocktail further enhances radon 
extraction from moisture containing charcoal. It has been empirically 
determined that a number of different radon extraction solvents can be 
usefully mixed with conventional LS cocktail counting solvents in 
volumetric proportions ranging from 2% extractiion solvent: 98% counting 
solvent to 90% extraction solvent: 10% counting solvent. A typical 
formulation for example employing ethanol extraction solvent utilized 20% 
absolute ethanol: 80% LS cocktail. The extraction solvents have been found 
to improve radon extraction between 10% and 500% depending upon the 
molecular weight of the aromatic solvent in the LSC cocktail and the 
moisture content of the charcoal. The presence of detergent, solubilizer, 
and emulsifier agents which are commonly included in LS cocktails used for 
counting aqueous samples does not interfere with the extraction of radon 
by the LMW organic solvents. Indeed, these said agents which increase the 
water-holding capacity of the LS cocktails may be helpful in the counting 
of radon-bearing charcoal samples in which significant moisture is 
present. Such moisture may be dissolved within the LS cocktail without 
decreasing the radioactive counting efficiency of radon. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention employs LS cocktails containing one or more radon 
extraction solvents (typically organic solvents having molecular weights 
of less than 100 daltons) for extraction of radon from charcoal. With 
certain of these solvents such as ethanol, radon extraction efficiency is 
improved, cocktail toxicity is absent or minimal, and usage and disposal 
of the cocktail and charcoal is facilitated and simplified. In choosing a 
radon extraction solvent to be used in an LS cocktail of the present 
invention the solvent should do the following. (i) Infiltrate the charcoal 
either alone or in combination with the other ingredients in the LS 
cocktail to remove, transport or otherwise release the radon gas into the 
LS cocktail which surrounds, or is in close proximity to the charcoal. 
This movement of radon from inside to outside the charcoal is necessary so 
that subsequently produced alpha and beta radioactive decay particles 
which travel very short distances in liquids, will successfully interact 
with the solvent and fluor molecules in the LS cocktail and be made 
visible as light flashes outside the charcoal particles. (ii) The radon 
extraction solvent should substantially sustain overall radioactive 
counting efficiency of the original LS cocktail which lacked the LMW 
organic solvent. That is, certain solvents which cause excessive 
"quenching" ie. attenuation of fluorescence light output from the cocktail 
are unacceptable. (iii) The radon extraction solvent should preferably be 
non-toxic or be of low toxicity to facilitate use and disposal of the LS 
cocktail. Alternatively the extraction solvent if toxic should be readily 
detoxified or otherwise eliminated for disposal reasons. (iv) The 
extraction solvent should preferably be miscible in the remaining 
ingredients of the LS cocktail so that radon extracted from the charcoal 
is communicated directly to the vicinity of fluor molecules in the 
cocktail where fluorescence may occur. If the solvent is only partially 
soluble (or insoluble) in the remaining ingredients of the cocktail (or is 
introduced to the radon-bearing charcoal which is near but not submerged 
in the LS cocktail), then the solvent must be one favoring radon chemical 
partition into the fluor-containing cocktail phase. This chemical 
partition is necessary for the radon and subsequent decay particles to be 
counted following interaction with the fluor molecules in the cocktail. 
(v) The extraction solvent should preferably be miscible or partially 
soluble in water and thus polar, to improve extraction of radon from moist 
charcoal. In the course of exposing charcoal-type radon detectors to 
ambient radon-bearing air, charcoal can adsorb between zero and 
approximately 35% by weight water. The amount of water depends upon the 
relative humidity of the air, the duration of exposure, and a variety of 
other parameters including the presence or absence of desiccant in the 
vicinity of the charcoal. It has been found that LMW polar extraction 
solvents substantially improve the extraction of radon from moist 
charcoal. LMW polar organic extraction solvents such as ethanol, by being 
soluble in both water and higher molecular weight organic solvent-based LS 
cocktails, assist in dispersing the water in the cocktail. In this regard, 
the present invention, by providing solvents and LS cocktails for improved 
extraction of radon from charcoal, represents a substantial improvement 
over the prior art. The previous toluene and other aromatic solvent-based 
LS cocktails exhibited relatively poor efficiency of radon extraction from 
moist charcoal, had a relatively high vapor and solvent toxicity and 
created substantial inconvenience and expenses in attempting to dispose of 
the LS cocktail waste products. These problems have been solved by LS 
cocktails of the present invention. The present invention additionally 
provides LMW polar organic solvents which by themselves are generally 
effective in displacing radon from charcoal.

EXAMPLE 1 
Low Molecular Weight Alcohols Extract .sup.222 Rn From Charcoal 
A number of LMW alcohols were added to a variety of commercial aromatic LS 
cocktails (manufactured by the Beckman and Amersham Corporations 
containing for example xylene, pseudocumene, dodecyl benzene or 
diphenyl-based solvents such as phenyl xylyl ethane, isopropyl biphenyl 
and methyl diphenyl ethane) and the efficiencies of radon extraction from 
activated granular charcoal were subsequently monitored. Radon extraction 
and radon daughter ingrowth equilibrium prior to LS counting of 2 g 
radon-bearing charcoal in 10 ml LS cocktail, (incubated at 23.degree. C.) 
was generally achieved in 6-8 hours. With proportions of alcohol to 
aromatic LS cocktail of between approximately 2%:98% and 90%:10% by 
volume, it was found that the LMW alcohols substantially enhanced the 
extraction of .sup.222 Rn from charcoal. At alcohol proportions above 
about 90%, the LS cocktail fluor may become excessively diluted or the 
alcohol may quench fluorescence to reduce radioactive counting efficiency. 
Proportions of alcohol to aromatic solvent between 5%:95% and 50%:50% were 
found particularly useful. The lowest molecular weight alcohols, methanol 
(32 daltons) and ethanol (46 daltons), were found to be most effective in 
extracting radon from charcoal (extraction efficiencies in decreasing 
order: methanol&gt;ethanol&gt;&gt;isopropanol) and were conveniently miscible in 
most of the commercial LS cocktails tested. Higher molecular weight 
alcohols such as propanol, butanol etc., showed progressively less ability 
to extract radon from charcoal. This observation suggests that one and two 
carbon alcohols are best capable of entering the smallest channels or 
pores in charcoal to displace .sup.222 Rn. Surprisingly the inorganic 
molecule water, which has even a lower molecular weight than methanol, was 
found to be poor solvent for quantitative displacement of .sup.222 Rn from 
charcoal. It is likely that the properties of water including its 
relatively high solvent viscosity, high surface tension, and relative low 
partition coefficient for .sup.222 Rn may be negative influencing factors 
in its radon extraction ability since an effective extraction solvent must 
be able to enter the finest pores in charcoal, solubilize the .sup.222 Rn 
and then diffuse out of the charcoal, carrying the .sup.222 Rn. 
Methanol and ethanol were found to be especially useful and effective for 
extracting .sup.222 Rn from moist charcoal. The toxicity and higher 
volatility of methanol reduce its desirability relative to ethanol 
however. Methanol and ethanol added to toluene, xylene, and other benzene 
derivative-based LS cocktails increased the amount of radon extracted from 
dry charcoal approximately 10% to 25% (methanol being the more efficient) 
and from moist charcoal approximately 40% (data not shown). Methanol and 
ethanol added in comparable proportions to diphenyl derivative and long 
chain alkyl benzene-based LS cocktails increased extracted radon levels as 
much as 4-5 fold over the solvent alone (See Example 2 and Table 1 below). 
EXAMPLE 2 Low Molecular Weight Alcohols Added to Conventional LS Cocktails 
Enhance the Extraction of .sup.222 Rn From Charcoal 
Two gram samples of activated charcoal which contained radon were mixed 
with 15 ml of the prior art conventional toluene-based LS cocktail or with 
15 ml of a diphenyl derivative solvent-based LS cocktail similar to that 
manufactured by Beckman Corp. and know as Ready Safe.TM.. Additional 
identical 2 g charcoal samples were added to 15 ml of the same LS solvents 
containing either methanol, ethanol or isopropanol in various percentages 
by volume. All samples were incubated at room temperature for 8 hours in 
20 ml capacity LS vials prior to counting. Table 1 presents average counts 
per minute (cpm) values (each value based on duplicate samples differing 
in cpm by .+-.2%) as well as extraction and counting efficiency percentage 
values (efficiency) determined by prior art normalizing the cpm values to 
that value obtained with the conventional toluene-base LS cocktail 
described by Pritchard and Marien (1983). It can be seen that radon 
extraction efficiency can be substantially improved over that of the prior 
art (assigned value of 100% in Table1). 
EXAMPLE 3 
Solvents Can Be Directly Added to .sup.222 Rn Bearing Charcoal to Displace 
the .sup.222 Rn 
Solvents having low molecular weights between approximately 30 and 100 
daltons (such as methanol, and ethanol), may be added directly to charcoal 
samples inside LS counting vials to displace the radon into the LS 
cocktail. For example 2 ml ethanol or methanol added to 2 g radon-bearing 
charcoal in a porous bag or a porous plastic canister suspended above 8 ml 
xylene-based LS cocktail (in a sealed LS counting vial) rapidly displaced 
the radon into the LS cocktail with high efficiency. The radon yield in 
the cocktail was 94% of that obtained by submerging untreated identical 
charcoal samples in the xylene-based LS cocktail which itself contained 
the same amount of alcohol. One advantage in suspending the charcoal above 
the LS cocktail rather than submerging it, is that any quenching of light 
output by charcoal dust in the LS cocktail can be essentially eliminated. 
Solvents which were relatively ineffective when mixed with LS cocktails in 
extracting radon from charcoal (eg. water and three carbon and higher 
alcohols), were also relatively ineffective in displacing radon when added 
directly to the charcoal. 
EXAMPLE 4 
Other Low Molecular Weight Organic Solvents Can Assist in .sup.222 Rn 
Extraction From Charcoal 
Ketones and ethers such as acetone and ethyl ether were tested as radon 
extraction solvents. Twenty percent by volume of ether, acetone or ethyl 
ether was dissolved in a standard xylene-based LS cocktail and in Ready 
Safe.TM. diphenyl derivative LS cocktail as described in Example 2. 
Negligible fluoresence quenching and moderately good extraction of radon 
from charcoal were measured. Furthermore, the same solvents functioned to 
displace radon directly from charcoal placed in proximity to LS cocktails 
within LS counting vials as described in Example 3. 
TABLE 1* 
______________________________________ 
LS Cocktail 
solvent cpm efficiency 
______________________________________ 
Toluene solvent 1196 (100%) 
+10% methanol 1507 125% 
+10% ethanol 1423 119% 
______________________________________ 
diphenyl derivative solvent+ 
(solvent alone) 359 30% 
+3% methanol 1224 102% 
+10% methanol 1525 128% 
+20% methanol 1536 128% 
+3% ethanol 767 64% 
+10% ethanol 1444 121% 
+20% ethanol 1450 121% 
+7% isopropanol 454 38% 
+30% isopropanol 608 51% 
______________________________________ 
*See Example 2 for explanation of this Table and definition of terms. 
+This solvent is the same as that used in the "Ready Safe" LSC formulatio 
of the Beckman Corp.