Introduction 40
1. Introduction 40
K decays by EC is always present. Therefore, if the background correction is made to the counts
In the later 80s, Mexico imported from an accumulated in the corresponding region of the
European country 28,000 tons of powdered milk
spectra, and the low background scintillation contaminated with the fission product Cs, due to
detection system is calibrated in order to establish the Chernobyl accident occurred few years before. the detection efficiency for γ radiation at 1461 KeV Since then, either for importing or exporting
specific energy, then it is possible to perform a very foodstuffs, it became compulsory in this country to
easy calculation and to get the K concentration either perform a low background gamma rays detection of
in powdered milk or any other liquid or conveniently the products, in order to guarantee the absence of
ground foodstuff. In this paper the results obtained in radioactive contamination by gamma emitters such
four basic diet products are given: milk (powder, as 137 Cs fission product. But in these detections the
40 light and cream), eggs (yellow and white), wheat 1461 KeV γ rays peak emitted by Ar when 11% of (whole and refined flour) and corn (whole and
Corresponding author: Juan Manuel Navarrete, Ph.D.,
cornstarch flour).
titular professor, research field: nuclear chemistry. E-mail: jmnat33@servidor.unam.mx.
Potassium Concentration by Natural 40 K- 40 Ar γ Rays Detection in Four Basic Diet Products (Milk, Eggs, 615
Wheat and Corn)
2. Experiment
= Isotopic abundance of 40 K (as percentage); 11/100 =
40 40 Nucleus of 40 K decaying by EC ( K → Ar) (as The potassium concentration in any foodstuff may
percentage); 0.693/1.28 × 10 9 × 365 × 24 × 3600 = λ of
be established through γ rays spectrometry when the
K expressed in s 9 (t 1/2 = 1.28 × 10 years) [1]; 74.5 = sample is put in a Marinelli container and detected molecular weight of KCl; 1 Bq = 1dps. several hours in a low background 3 × 3” NaI (Tl) However, it is also possible to use a multinuclide scintillation crystal surrounded by heavy shielding, and standard source, such as those produced by Isotope coupled to a PC charged with the Maestro Program for Products Laboratories in California, with several radioactive detection. Marinelli containers used were radioisotopes distributed homogenously in an epoxy 500 mL total volume in 1.5 cm width and 7.5 cm height matrix, filling an annular space of exactly the same annular space. dimensions than the Marinelli containers used to detect Detection time for sample charged and empty the natural radioactivity in the samples [2]. Even when Marinellis (background) were from 12 to 24 hours. these sources are very useful to calibrate γ spectra and In order to determine the detection efficiency of the to establish detection efficiencies at similar conditions, system for 1461 KeV
40 -1
γ rays emitted by 40 Ar nucleus in the case of natural radioactivity, such as 40 K
during 11% of 40 K nucleus decaying, the easiest detection, they present some problems to be solved
procedure is to charge a Marinelli container with a such as large corrections for dead time and much
volume of KCl (AR) exactly the same than that used to higher Compton distribution in 1 minute detection,
detect the samples, just to the top of annular space. This while the expressly made KCl source results cheaper,
volume of KCl is weighed in a common balance and more similar to foodstuffs and consequently more
detected during 10 minutes. The counts accumulated in precise for obtaining 40 K detection efficiency in 10 the 1461 KeV peak are expressed in counts per second
minutes detection time. Also, it makes possible to use (Bq) (11% of 40 K nucleus decaying by EC) and
different detection geometries (different sample height corrected for background taken in the same time. This
and volume in the Marinelli container) provided that correction is almost unnecessary due to the high ratio
detection conditions be kept identical for sample and of counts per second produced with the Marinelli
KCl standard. Fig. 1 shows a plot of Detection charged with KCl compared with the counts obtained
Efficiency (%) vs. Height of KCl sample in Marinelli with the empty Marinelli (background). When the
container (cm).
cps 40 K- 40 Ar are divided by the KCl weight, In this way, the total activity of 40 K (11% of the cps 40 K- 40 Ar/g KCl are obtained under our detection
nucleus decaying to 40 Ar by EC plus the 89% of the conditions. If these counts are divided by the constant
nucleus decaying to 40 Ca by β-emission) per gram of
40 specific activity of KCl (dps 40 K- Ar/g KCl) due to foodstuff sample is obtained from the next equation: 11% of 40 K nucleus decaying by EC, the detection
A F 40 (dps K/g of sample or Bq 40 K/g of sample) efficiency for 1461 KeV
( W F )( D E )( 0 . 11 γ rays in our particular )
Where: C S = Counts per second (1461 KeV peak) detection system is obtained. The specific activity of
40 from foodstuff sample (12-24 h detection time); C KCl due to = 40 K- Ar decay (1.8 Bq/g KCl) is calculated B Counts per second (1461 KeV peak) from background
from the next equation:
( 6 . 02 × 10 Sp 23 . Act . KCl ( K Ar ) )( 0 . 0118 )( 11 )( 0 . 693 )
radiation (12-24 h detection time); W F = Foodstuff ( 74 . 5 )( 100 )( 100 )( 1 . 28 × 10 9 )( 365 )( 24 )( 3600 ) sample weight (g); D E = Detection efficiency in our
detection system for 1461 KeV; 0.11 = 11% of 40 K
= 1 . 8 Bq 40 K − 40 Ar / gKCl
nuclei decaying by EC to Ar ( γ rays emitter, 1461 Where: 6.02 × 10 23 = Avogadro’s number; 0.0118/100
KeV).
616 Potassium Concentration by Natural 40 K- 40 Ar γ Rays Detection in Four Basic Diet Products (Milk, Eggs, Wheat and Corn)
y = -0.11x + 3.76 R 2 = 0.9912
Height of KCl sample in M arinelli (cm)
Fig. 1 Graph of Detection Efficiency% vs. Height of KCl sample in Marinelli (cm).
The total specific activity of elementary K, or total
Table 3 K concentration in wheat (%).
activity of 40 K per gram of elementary K, is obtained
Refined wheat flour from the next equation:
Whole wheat flour
= 31 . 19 Bq 40 K / gK Table 4 K concentration in corn (%).
40 Whole corn flour
Cornstarch flour
Therefore, when the result A F (Bq K/g of sample) is
0.018 ± 0.0009 divided by the constant value A K = 31.19 (Bq K/g K),
the K mass per gram of sample is obtained as C (g K/g
4. Discussion
of sample) [3]:
40 A It has not been applied any correction factor for self (Bq K/g of sample)
C(g K/g of sample) = F 40 absorption, which must not exist with so penetrating γ
31.19(Bq K / gK )
rays and materials with similar densities around 1 g/mL
3. Results
in 1.5 cm of linear thickness. On the other hand, if it is possible to find some foodstuffs with lower density
Tables 1, 2, 3 and 4 show the results obtained for K than 1 g/mL, such as powdered milk and amaranth
concentration (%) in milk (powder, light and cream), seeds, any other nourishing product conveniently eggs (yellow and white), wheat (whole and refined
ground or liquefied must have a density not very much flour) and corn (whole and cornstarch flour) respectively.
greater than 2 g/mL, and so it is considered not
Table 1 K concentration in milk (%).
necessary any correction for self absorption dealing Powder milk
with foodstuff samples such as honey and flours, for 0.63 ± 0.03
Light milk
example. Nevertheless, the different Compton scattering for different sample densities might be a
Table 2 K concentration in eggs (%).
problem to solve, but again the densities range for Yellow White
foodstuffs is limited, and so the differences in counting due to different densities in samples must be included
Potassium Concentration by Natural 40 K- 40 Ar γ Rays Detection in Four Basic Diet Products (Milk, Eggs, 617
Wheat and Corn)
in the statistical variation ± 5%, addition of counts may be used as a measurement of the mineral richness of variation for sample and background for detection
every foodstuff, since potassium must come primarily times from 12 to 24 hours. For repeated determinations
from minerals in the soil, as well as a way to test the in the same sample, it has been obtained too σ n-1 = ± 5%,
efficiency of fertilizers in grounds producing any kind of for n = 5 detections and mean value = 1.78 ± 0.09% of
nourishing products. This technique is definitely easier K for arabica variety coffee from Coatepec region in
and simpler than any other determination of potassium Mexico. This result has been compared with that
by spectroscopy, since in spite of long detection times, it obtained in same coffee variety from 6 regions in
does not require any previous conditioning of the sample America and Africa with a total of 32 analytical
but to grind or liquefy in some cases, compared to more determinations by flame photometry, whose average
complicated sample preparation needed by AAS or value was = 1.72 ± 0.1% [4]. Also, the possible
ICP-AES, for example. Moreover, it may be the natural
contribution of 40 Ac (1459 KeV) to the K peak (1461 base for any possible study of radioactivity in foodstuffs. KeV), has not been considered in the produced spectra,
The uncertainty ± 5% comes from the sum of because being a member of the 232 Th radioactive
uncertainties in the counts accumulated for background natural chain, the presence of 208 Tl (2614 KeV) peak
and samples detection, but of course may be reduced to a should be unavoidable, and it has not appeared in any
lower limit by enlarging the detection times. Besides, the foodstuff detected. Besides, the presence of heavy,
technique is so simple and comprehensive that it is radioactive metals such as 232 Th, 235 U and 238 U and
possible to perform in any laboratory with a minimum of their chains are common in raw materials and mineral
facilities beside one low background scintillation samples such as sand, mortar, plaster, brick, rocks,
detector system. Also, it has been successfully used as a soils and zircon [5], but quite unusual in foodstuff. laboratory experience for undergraduate and graduate
Therefore, the results obtained seem to be precise students, in order to illustrate with just one experience so enough between the limits of statistical variations.
many subjects as natural radioactivity, radioactivity in Nevertheless, the results presented in Tables 1, 2, 3 and
foodstuff, radioanalytical chemistry and background
4 might be obtained with shorter statistical variations
radioactivity, for example.
inherent to radioactive detection, if detection times for