Gross Alpha

8/12/2019 Gross Alpha

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GROSS ALPHA ACTIVITY MEASUREMENT PROCEDURES

(Ref: Indian Standard: IS 14194 (Part 2): 1994 "Radio-nuclides in Environmental

Samples - Methods of Estimation)

Objective:To estimate Gross - Alpha activity levels in environmental samples, Including air,water etc.,

Principle:The gross alpha activity of the sample, either particulate collected on an air filter,

or water (concentrated or precipitated) and dry deposited on metal Planchet (stainless steel) is

directly measured in an alpha counter, having Zinc Supplied (Ag) scintillation detector.

Apparatus: i) Alpha counting system having a Zinc Supplied (Ag) detector equipped with a

sample holder to accormnodate stainless steel planchets placed in an aluminum planchet

holder. The background of the system shall be less than 5 counts per minute (CPM) and

counting efficiency, >20%.

ii) Stainless steel planchets: 0.5mm thick and 25mm in diameter, (placed in an aluminum

planchet holder).

iii) Infra-Red Lamp: about 500 Watts, (used for drying precipitates before counting).

PROCEDURE:

AIR FILTER:Working atmosphere or ambient atmosphere may contain manmade redionuclides or natural

radioactivity arising from technological operations or purely from natural environmental

processes. Although a certain level of radioactivity in the air we breathe may not be harmful

but at levels beyond the permissible limits, this could pose potential hazards to individuals atsuch locations. Hence, the need to monitor and evaluate the radioactivity levels in air in such

environments.

In this procedure, we are generally talking of the naturalrationuclides in the atmosphere, of

which some have short half-life e.g., Radon and daughters while some have long half-lives

(e.g. nat. U nat. Th, Ra-226, Po2lO etc). Presently we are concerned only with the long-

lived component of the alpha activity. The procedure for estimation of long-lived alpha

activity in air filters is as follows:

a) Place a glass fiber or Whatman-40 or Millipore filter sample (25mm dia) in the suction

head of an air sampling pump, and secure it firmly on the suction head without allowing

any leakage.

b) Note the time and switch on the pump. Not the air flow rate in the flow meter. Filterabout 5M3of air.

c) Stop the pump, and note the time and calculate the volume of air passed (in M3) through

the filter paper, using the flow meter and pump running time.

d) Remove the filter containing the particulates in the sampled air, from the suction head,

and store for 300 min. (to allow short lived random daughters to decay)

e) Subsequently, count the filter paper containing the particulates for alpha counting, by

placing it in an aluminum planchet holder, in the Alpha-Counter, which is kept on for

about an hour, prior to counting.

f) Counting for a reasonable period, say around ~lO0 min., and note the activity in counts

per minute (CPM) and calculate the Gross, long lived alpha activity of the sampled air,

by using the counting efficiency of the alpha counting system (in %) and volume of airsampled (M3).


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The Gross alpha activity (AB) of the sample is calculated as follows:

Count rate of the sample mounted on the planchet (Cs) in counts per minute (CPM) = NS

NB. Where Ng = Count Rate in CPM of the sample + count rate of background in CPM.

NB Background count rate in CPM. The Gross alpha activity of the sample (As) in Bq/ M3of

air is = Cs x 100/E x l/V,

Where, Cs = Net count rate of the sampling in CPM.

E = Counting efficiency of the alpha counter in percent.

V = Volume of air filtered in M3.

Water:Environmental waters, whether they belong to fresh water (e.g., rivers, lakes, ponds, ground

waters) or saline/brackish water category (e.g., seawater, estuarine waters), all contain some

dissolved radionuclide components, arising from ecosystem processes, due to physico

chemical, geo-chemical and biological activities taking place amongst ecosystem

components. Some of the natural alpha redionuclides of interest to human health & safety arenat. Uranium, nat. Thorium, Ra-226, Rn-222 (this is a gaseous one, and hence not significant,

for drinking water, considerations). Po2l0 etc. Amongst these, generally nat. Uranium, Ra

226 & nat. Thorium may be considered of interest, as their levels have been found to be

spread over a wide range of concentrations in potable waters, world wide including in bottled

mineral waters and packaged drinking waters. In the light of the above, it becomes highly

relevant to assay the levels of the above alpha emitting redionuclides in water samples,

commonly grouped under the term grossalpha activity". The procedure for the analysis of

gross alpha activity is given below.

Procedure:l. For water samples, which has, a low Total Dissolved Solids (TDS) content; say less

than 100 ppm.

a) Check the TDS content of the sample under test, by standard laboratory methods.

b) If the test results, demonstrate the TDS level at


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technique to isolate the desired alpha activity containing radionuclides from the samples

followed by their counting in an alpha counting system as above.

Regents required:i) Bismuth Carrier, prepared from a standard A.R. grade Bismuth Nitrate, Bi (NO3) 3, at a

concentration level of 50mg Bi/ml.ii) Lanthanum Carrier, prepared from a standard A.R. grade Lanthanum Nitrate, La (NO3)

3at a concentration level of lmg La/ml.

iii) Phosphoric Acid, (Cone.)H3PO4.

iv) Hydrofluoric acid - Nitric acid wash soln. : Mix.5ml Hydrofluoric acid and 5ml nitric

acid and dilute to l litre with dist-water.

Procedure:

l. Alpha background activity determination:

Although we use the best of chemical reagent standards, whose chemical compositionand assay may not be questionable, but they may still carry traces of natural radioactive

nuclides. For e.g., lanthanum reagent is known to carry traces of radio activities due to

nat. Thorium, nat. Uranium etc., because they generally co-occur in their geological

origins and sources. Hence, it becomes essential to check and evaluate the extent of nat.

Uranium, nat. Thorium presence in the lanthanum carrier, when we are employing the

same for gross alpha determinations. The procedure for carrying out the same is as

follows:

a) Transfer accurately by pipette 1 ml lanthanum carrier into a polythenecentrifuge

tube (50ml capacity), dilute to 5ml with 4N Hydrofluoric acid and stir well.

b) Add 1 ml Conc. Hydrofluoric acid slowly and stir well with a Teflon (or PVC) rod

and centrifuge for about 5 minutes.

c) Pour out the supernatant and reject the same. Wash the residual precipitate in the

centrifuge tube with Hydrofluoric - Nitric acid wash soln., centrifuge again for

about 5 - 10 minutes and reject the supernatant.

d) Wash the precipitate in the centrifuge tube with 5ml dist. Water and centrifuge

again. Reject the supernatant.

Note: The supernatant should be free from acidity

e) Add a few drops of dist. Water to the precipitate residue in the centrifuge tube, stir

well and completely transfer the precipitate - slurry on to a clean stainlessplanchet, with the aid of a dropper.

f) Dry the precipitate slurry on the S.S. Planchet under an infrared lamp, taking care

to uniformly spread the slurry on the planchet, (to minimize self- absorption errors

of alpha activity).

g) Flame carefully the dried residue on S.S. Planchet, on a laboratory burner. Cool

and count the same in the alpha counting system, and record the lanthanum

Fluoride background count rate NlaF3). Caution: Ensure that during flaming the

residue does not flake off and get lost.


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Alpha activity determination in Water Samples

a) Transfer l litre of filtered water sample to a beaker, add 5ml Conc. HNO3, and 500mg

Bismuth carrier (i.e., ml of Bi carrier soln) with stirring.

b) Add lg of Hydroxylamine Hydrochloride (NH2OH.HCI) and stir again.

c) Now, warm on a hot plate and add 1 ml phosphoric acid with stirring, when BismuthPhosphate (BiPO4) starts precipitating. Allow to cool, when (BiPO4) precipitation is

complete, (about an hour may be required). The BiPO4 precipitate will carry most of

the alpha emitters, under study.

d) Now slowly siphon off the supernatant, and transfer the precipitate as slurry, to a

polythene centrifuge tube (50ml capacity)

e) Centrifuge the above for a minute and discard the Supernate.

f) Dissolve the BiPO4 precipitate in the centrifuge tube in Conc. HCI, of adequate

quantity. Now add 1 mg lanthanum carrier (i.e., 1 ml of Lanthanum Carrier soln),

followed by slow addition of Conc. Hydrofluoric Acid (HF). Slowly stir with a

polythene or PVC or Teflon rod, when Lanthanum Fluoride (LaF3) precipitates. Allow

some time (10-15 minutes) for completeness of precipitation.g) Centrifuge and discard the supernate.

i. Wash the precipitate thoroughly in the centrifuge tube with Hydrofluoric Nitric

acid wash solution. Centrifuge and discard supernate.

ii. Wash the precipitate in the centrifuge tube with 5ml distilled water, centrifuge and

discard supernate. Note: The supernatant should be free from acidity.

iii. Transfer the precipitate from the centrifuge tube completely with few drops of

dist. Water as slurry onto a stainless steel planchet using a dropper.

iv. Dry the S.S. Planchet (containing the LaF3 precipitate) under an Infrared lamp,

taking care to uniformly spread the slurry on the planchet (to minimize self-

absorption errors of alpha activity).

v. Flame carefully the dried residue on the S.S. Planchet on a laboratory burner, cool

and count the same in the Alpha Counting System.

h) Record the count rate of the sample (together with background count rate), (Ns).

i) Now subtract the count rate of background (NLaF3 ;) from the gross count rate of the

sample plus background (Ns) count rate, to get true count rate of the sample. The true

count rate of the sample mounted on the planchet (Cs) in counts per minute (CPM) is as

follows:

Cs = Ns - (NB+ NLaF3) (CPM)

Where,

Ns = Count Rate (in CPM) of the sample plus count rate of Background (in CPM)

NB= Background count rate of counter (in CPM)NLaF3= Blank LaF3count rate (in CPM)

j) Calculate the gross - alpha activity of the sample (As) under study as follows:

As = Cs x 100/E x 1/S,

(Bq/1)

Where

Cs = Net count rate of the sample (in CPM)

E = Efficiency of the alpha counting systems (in o/o)

S = Sample size taken for the analysis (1 litre)

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