The National Physical Laboratory 7-ionization chamber has been used for the measurement of chemical yield by re-irradiation in a method for the determination.
Journal of Radioanalytical Chemistry, Vol. 10 (1972) 245--256
Physical Methods Section USE OF THE NATIONAL PHYSICAL LABORATORY IONIZATION C H A M B E R T Y P E 1383A 1N N E U T R O N A C T I V A T I O N A N A L Y S I S
K. HEYDORN Isotope Division, Danish Atomic Energy Commission Research Establishment Riso, Roskilde (Denmark) (Received July 16, 1971) The National Physical Laboratory 7-ionization chamber has been used for the measurement of chemical yield by re-irradiation in a method for the determination of arsenic by neutron activation analysis. Satisfactory accuracy and a precision of 1 were obtained with a single reading. Discrimination against other radioisotopes is achieved by irradiating for a short time and measuring at a total decay time of one mean life of the radionuclide to be determined; the resulting discrimination factor is given as a function of the half-life ratio. Maximum sensitivities for 66 elements with y-emitting thermal neutron capture products were calculated for irradiation and decay times both equal to one half-life, and it is shown that the sensitivity for shorter irradiations:/tt- the selected time of measurement is a linear function of the irradiation time. More than a dozen elements were found suitable for determination at the milligram level by neutron activation followed by 7-ionization chamber measurement. The error from interfering elements can be directly estimated from their expected concentrations by means of the calculated sensitivities and discrimination factors presented in the paper.
Introduction A l t h o u g h a considerable n u m b e r of different radiation detectors have been applied in the course of time for n e u t r o n activation analysis measurements, the y-ionization c h a m b e r seems to have attracted little or no a t t e n t i o n since its use by LEDDICOTTE1 twenty years ago. M e a s u r e m e n t of i o n i z a t i o n current yields a direct d e t e r m i n a t i o n of the q u a n t i t y o f radioactive material in a n irradiated sample at a given time, but rather little i n f o r m a t i o n on its composition. The accuracy of m e a s u r e m e n t is excellent over a wide range from the level of n a t u r a l b a c k g r o u n d radiation, b u t adequate precision usually requires activities in the pCi range. In samples of k n o w n composition the ionization c h a m b e r is a useful i n s t r u m e n t for the quantitative d e t e r m i n a t i o n of a n u m b e r of elements at and above the milligram level by n e u t r o n activation analysis, where the use of more sensitive detectors requires a particular, low-level irradiation, or a reduction of sample size. J. Radioanal. Chem. 10 (1972)
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Low-level irradiation can be effected by irradiating in a special, low-flux facility or for a very short period, but in many cases operation of the reactor at a reduced power level becomes necessary. Neutron activation analysis determination of trace elements relative to a major element in the same sample usually requires two different irradiations if the same detector is used throughout. Reduction of sample size may not be possible because of lack of homogeneity in the sample material; in other cases a sample must be analysed in its entirety, because subdivision might jeopardize the validity of the analytical results. The ionization chamber thus appears to be an interesting alternative for the determination of major elements in neutron activation analysis. Instantaneous readings from an ionization chamber are well suited for decay curve analysis, but in many cases proper selection of the irradiation and decay times will permit accurate determination of an element in a single measurement. Specifically in the determination of chemical yield by re-irradiation, where the composition of the sample is under strict control, a single reading will usually be adequate.
General considerations
For the present investigation the re-entrant y-ionization chamber type 1383A designed by the National Physical Laboratory 2 was selected. This instrument is available in many laboratories, and its properties are well documented in the literature; z-4 variation in 7-response between individual chambers is within _ 1 ~o of the reference at the National Physical Laboratory.
Choice of irradiation and decay time As in any other instrumental analytical method, selection of the optimum conditions for the determination of a particular element requires a priori information on the composition of the sample. When such information is available, deviations from the expected concentration of a particular element can be determined with optimum precision: 'n In the general case only properties of the element to be determined can be taken into account in the selection of proper conditions. By thermal neutron irradiation of a sample for a time Ti, the activity of a radioisotope at the time T c after the end of irradiation is A = a(1 - e -zT') e -zTc where a is the saturation activity and ,~ the decay constant. J. Radioanal.
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For fixed T~ and T e the activity varies with half-life so that 6A 62- = a e -aT~ [(Ti + Tc) e -zTi - To]
(2)
and the activity of an isotope with 2T i=ln
l+Tc
(3)
is maximized relative to isotopes with shorter or longer half-lives. 7 In the special case of T i = T c = z the isotope with half-life z is maximized. The influence of half-life on the activity can be enhanced by maximizing 6A/6 2 with respect to Ti 62A 6T -2-6~ . = a e -a(T~ + T,)[1 -- 2 (T~ + Ti)] = 0 for
(4)
2 T i = 1 - 2To.
(5)
By elimination of T~ from Eqs (3) and (5) we find 2 T i = - I n (1 - 2Ti)
(6)
which shows that Ti must be small in comparison with the half-life of the isotope to be determined, while T i + T c equals 2 -1. Under these conditions Eq. (1) is reduced to 2T;
A = a---
(7)
e
If T i is a small fraction f of the half-life z, Eq. (7) becomes A=fa
2T a In 2 =f ___~_fa e e 4
(8)
In the special case of Ti = Tc = z, Eq. (1) becomes A* = a/4,
(9)
A ~-fA*
(10)
which shows that for Ti = f z The graph in Fig. 1 presents A/A* as a function of f = Ti/~ for 6 A/6 ,~ = O, and illustrates that the approximate Eq. (10) holds within 2 % up to f = 1, i.e. T; = z. The validity of Eq. (5), however, is restricted to small values o f f J. Radioanal.
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K. HEYDORN: NATIONAL PHYSICAL LABORATORY IONIZATION CHAMBER
In practice this means that the shortest irradiation time needed to produce a desired ionization current from the expected quantity of an element at the time of measurement, can be immediately determined from a table of ionization currents from the activity A* per unit weight of the elements. A~ 4.0
01
O.1
0.01
[ 0.01
0"0010".001
4
E 1.0
0.1
I
I~_
10
F i g . 1. R e l a t i v e a c t i v i t y a t t h e s e l e c t e d t i m e o f m e a s u r e m e n t life i r r a d i a t i o n
Ti)TO0
as a f u n c t i o n o f f r a c t i o n a l h a l f -
Half-life discrimination A short irradiation time T i followed by a decay time T c = 1/2 - T i provides discrimination against radioisotopes with decay constants 2j different from 2. For
T i = f z = fj zj
(11)
according to Eq. (10)
A=fA*
at
1 Tc=~(-T
Aj=fjA~
at
T c + AT c.
i
(12) (13)
At the time Tc the ratio between the activities becomes Aj = Aj*
A
. f_~j e;~,aTe.
A*
(14)
f
For .fj also small, Eq. (5) is fulfilled, and 1
ATe- ,Zj J. Radioanal. Chem, 10 (1972)
1
2
"
05)
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CHAMBER
N o w Eqs ( l l ) and (14) become Aj -A
A*, = - A*
~ 9
1-
--e "rj
(16)
~i
The graph in Fig. 2 shows the discrimination factor qj----e
~J as a function o f - - - .
"Cj
"~
qj~l 1.0-
0.1
0.01
0.001 0.05 0.1
1.0
10
100
1000
[ 5000 ri/r
Fig. 2. Half-life discrimination at the selected time of measurement for short irradiation times For factor 4/e = For
increasingfj Eq. (15) is not applicable, but the effect on the discrimination is comparatively small; for fj = 1 the factor qj should be multiplied by 1.47. increasing rj, the factor qj becomes a linear function of the half-life ratio T qj---~ e - - . rj
Sensitivity
calculation
The sensitivity of the m e t h o d for a particular element is expressed in terms of the ionization current per unit weight produced under given irradiation conditions. Irradiation at a thermal neutron flux of 1013n 9 cm -z 9 sec -1 for one half-life, followed by a decay period of one half-life, results in an activity of A* = -
a
4
per milligram of element, where a - - 1.627
fly
M
mCi.
Here M is the chemical atomic weight of the element, and v is the percentage abundance of the activated nuclide with the thermal neutron activation crosssection a barn. Y. Radioanal. Chem. 10 (1972)
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K. HEYDORN: NATIONAL PHYSICAL LABORATORY IONIZATION CHAMBER
Table 1 Calculated m a x i m u m sensitivity for elements irradiated for one half-life at a thermal n e u t r o n flux of 1013 n " c m - 2 - sec -a, m e a s u r e d in the N a t i o n a l Physical L a b o r a t o r y Ionization C h a m b e r Type 1383A after one half-life decay Element
Air Aluminium Antimony Arsenic Barium Bromine Cadmium Cesium Calcium Cerium Chlorine Chromium Cobalt Copper Dysprosium Erbium Erbium Europium Fluorine Gadolinium Gallium Germanium Gold Hafnium Holmium Indium Iodine Iridium Iron Lanthanum Lead Lutetium Magnesium Manganese Mercury Molybdenum Neodymium Nickel Niobium Osmium Palladium Platinum
J. Radioanal. Chem. 10 (1972)
Radionuclide
~lAr 2SA1 124Sb
TeAs 13aBa mBr 117mCd/llTln
134Cs 49Ca 141Ce
3sC1 ~lCr eoCo 64Cu
~eSDy leTmEr 171Er 152EH 2oF
l~'Gd 72Ga 75,nGe lasAu 17amHf 16eHo 11em1In 12sI
19~ir SaFe 140La 2oT,,pb 177Lu 27Mg
Z6Mn 2~ 9aMo/gamTc 149Nd 65Ni a4Nb 19XOs 10apd ~gapt
Half-life
1.83 h 2.3 m 60.3 d 26.4 h 84.0 m 35.4 h 3 h 2.10y 8.7 m 32.8 d 37.2 m 27.7 d 5.24 y 12.81 h 139 m 2.4 s 7.52 h 12.5 y 11.2 s 3.7 m 14.1 h 47.1 s 2.69 d 18.6 s 27.0 h 55.0 m 25.0 m 74.3 d 45.0 d 40.2 h 0.8 s 6.74 d 9.5 m 2.58 h 47.1 d 66.6 h 1.9 h 2.54 h 2.0 - 104 y 15.2 d 13.6 h 30 m
Ionization current, pA/mg
0.20 11.8 15.1 21 0.25 51 2.4 400 0.11 0.35 5.2 0.46 1 440 9.8 154 3.9 3.9 24 000 0.74 0.49 65 0.15 220 27 12 2 900 3.5 1 780 0.065 113 0.05 20 0.10 380 1.6 0.65 0.68 0.015 18.5 1.7 0.45 0.53
K. HEYDORN: NATIONALPHYSICAL LABORATORYIONIZATIONCHAMBER
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Table 1 (cont.) Element
Potassium Praseodymium Rhenium Rhodium Rubidium Ruthenium Samarium Scandium Selenium Silver Sodium Strontium Sulfur Tantalum Tellurium Terbium Thorium Thulium Tin Titanium Tungsten Vanadium Ytterbium Yttrium Zinc Zirconium
Radionuclide
42K 142pr
lSSRe lO4Rh S6Rb t03Ru 153Sm 46Sc 77roSe
llOmAg '-,4Na 87mSr z78 lSZTa lnlTe 16OTb 233pa 17OTto 125Sb ~ITi 187w
5eV l~ay b aomy 65Zn ~sZr
Half-life
i
12.5 h 19.2 h 17.5 h 43 s 18.7 d 39.6 d 46.7 h 84.1 d 18 s 252 d 15.0 h 2.84 h 5.1 m 115 d 24.8 m 72.5 d 27.2 d 130 d 2.52 y 5.8 m 23.9 h 3.75 m 31.8 d 3.1 h 246 d 64.9 d
Ionization current, pA/mg
0.55 4.1 12.5 17.5 0.72 2.2 32 930 4.8 35 76 0.62 12- 10 -~ 127 0.28 133 5.3 8.1 0.05 0.065 25 115 44 0.01 1.76
0,11
Values of a are t a b u l a t e d in the literature 8 a n d were checked by c o m p a r i s o n with the cross-sections given by GOLDMAN a n d ROESSER.a The i o n i z a t i o n currents pA* can be calculated from the c h a m b e r response f u n c t i o n derived by DALE3 a n d the 7-emission rates given by LEDERER et al. 1~ C h a m b e r response was n o r m a l i z e d to 6~ at the m i d p o i n t of the cavity, 4 a n d parabolic i n t e r p o l a t i o n was used between the tabulated values of the response function. F o r each element the isotope giving the highest sensitivity p A ~ was selected, a n d in Table 1 results are presented in p A / m g at 760 m m Hg a n d 22 ~ Estimation
of error
Interfering elements give rise to an error, which can be expressed as the percentage of the i o n i z a t i o n c u r r e n t p A from the element to be determined at a concent r a t i o n C, produced by other elements present in the sample. J. Radioanal. Chem. 10 (1972)
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K. HEYI)ORN: NATIONAL PHYSICAL LABORATORY IONIZATION CHAMBER
Interference from a particular element with the concentration Ci is
Ii = 100 _Ci _ C
.
~i pA* qiJ ~ < 1 0 0 Ci -pA ~
C
pA,? ~ qu pA ~
where summation with respect to j takes into account the various radionuclides produced from the same element. In practice 2qij will be < 1, and interference from elements with pA]/pA ~ < 0.01 J
may be neglected unless present in higher concentrations than the element to be determined; similarly, elements present in concentrations Ci < 0.01 C may be disregarded unless their tabulated sensitivity p a p is higher than pA ~ For the rest, qi is easily estimated from the graph in Fig. 2, and if necessary pA* can be calculated for the most interfering nuclides. In this way a reasonable estimate of the total error 2;Ii is obtained, and justification for the use of the ionization chamber for the determination of a particular element carl be ascertained, or correction can be made.
Experimental
Measurement of ion&ation current Ionization currents are measured with a polarizing battery voltage of 90 V as the voltage drop across a 100 Gf2 resistor by means of a Vibron electrometer* connected to a digital voltmeter to facilitate instantaneous readings. The capacitance of the ionization chamber with a one-metre cable is 150 pF, and the input capacitance of the converter unit is 45 pF, while the exact value of the resistor was measured at 1.02 9 10 ~1 f2. The speed of response expressed by the time constant becomes RC = 20 sec. The unshielded background at the measuring location averages (1.0_0.1) 9 9 10 -14 A at 760 m m Hg and 22 ~ and fluctuations in the readings corresponded to a standard error of measurement of 1.0 9 10 -15 A. In order to make background fluctuations insignificant, ionization currents of ~ 2 9 1 0 - 1 3 A** are required for an irradiated sample. Samples are placed in a perspex sample holder with their centre at the midpoint of the chamber at least 3 min before the time of measurement, and the background is checked 3 rain after removal of the sample. Since T c ~< 1/2, only radioisotopes with a half-life z > 125 sec can be measured properly, and in addition short half-life discrimination deteriorates as the speed of response assumes importance. * Electronic I n s t r u m e n t s Ltd., Model 33 C. ** This c o r r e s p o n d s to less t h a n 10/zCi of a 1 MeV ?-emitter, which can be handled in any good chemical l a b o r a t o r y . 1. ~Radioanal. Chem. 10 (1972)
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Arsenic yield determination Determination of arsenic in biological material by neutron activation analysis requires chemical separation followed by measurement of the chemical yield. Re-irradiation yield determination is used in a method for the determination of arsenic in blood 11 and a comparison was made between yields found by counting with a NaI(T1) scintillation detector and by measuring ionization currents in the National Physical Laboratory ionization chamber. The counting samples contain about 1 mg of arsenic carrier dissolved in a m m o nium sulfide solution. Carriers of copper and antimony were added in the chemical separation, and from experimental interference data 12 their concentrations relative to arsenic can be estimated at well below 1 ~ , while other impurities would be present only at the trace level. Estimation of interference from the elements in question made by means of the calculated sensitivities in Table 1, indicates that no significant error would be expected in the determination of arsenic by ionization chamber measurement of the irradiated samples. The samples were irradiated along with a 1 mg arsenic comparator sample and an equal volume of a m m o n i u m sulfide blank in sealed, half dram polyethylene vials for 20 rain at a t h e r m a l n e u t r o n flux of 7 1012 n 9 cm -2 9 sec -1 in the Danish reactor D R 2 to produce an ionization current of about 2 9 10 -13 A. The irradiated samples were measured in the ionization chamber at 1/2 - T i ~ 38 hrs after the end of irradiation, and yields were calculated after correction for blank and background. With less than 10 min between measurement of sample and comparator, decay corrections could be neglected together with corrections for temperature and pressure. Measurements on 16 blank samples gave an average of 0.5 9 10 -14 A, and blank variation was insignificant compared with the background fluctuation; a constant value could therefore be assigned to the blank, equivalent to 0.02 mg of arsenic at the time of measurement. 9
Precision and accuracy Measurements on 10 different samples, each with ten consecutive readings at 20 sec intervals, gave a standard deviation for a single reading of 0.14 9 10 -a4 A. With an average ionization current of 2.5 9 10 -13 A and a background variation of 0 . 1 0 . 10 -14 A, yield determinations based on single readings have a relative standard deviation of 1 ~ . Accuracy was tested by comparison of the ionization chamber results with the yields determined by counting for 4 min with a 3"• 3" NaI(TI) scintillation detector connected to a 511-channel pulse-height analyser at about three days after irradiation. The difference in chemical yield determined by ionization chamber measurement with ten readings per sample, corresponding to a standard deviation of 0.3 ~ , and by 7-spectrometry of the 0.56 MeV photo-peak of 78As with a standard J. Radioanal. Chem. 10 (1972}
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K. H E Y D O R N :
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deviation of 0.5 % based on counting statistics, averaged 0.6% absolute for 20 different samples covering the range of 80 to 100 ~oOn the assumption that the 7-spectrometric determinations are unbiased estimates of the true chemical yield, ionization chamber measurements exhibit a positive bias at the 1% level of significance, which is in accordance with the inherent tendency of this method. For all practical purposes an average error of this magnitude is insignificant, and the accuracy as well as the precision of the ionization chamber measurements are entirely satisfactory.
Discussion The use of an ionization chamber in neutron activation analysis is in many respects analogous to the use of a balance in gravimetric analysis; the simplicity of the equipment and its ability to measure accurately over a range of many decades, as well as the speed of measurement and its direct conversion into analytical results, are common characteristics. Knowledge of the chemical composition of the sample material is crucial in both types of analysis, which means that in most cases the measurement must be preceded by a chemical separation. However, the influence of impurities is markedly different, since the ionization current depends not only on their quantity, but also on their nuclear characteristics. Elements to be determined by neutron activation followed by ~-ionization chamber measurement must have a reasonably high sensitivity and a comfortable half-life of the radionuclide. In Table 2 are listed such elements and their radionuclides, selected according to the following criteria: (a) A quantity of 1 mg should produce an ionization current of not less than 2 9 10 -13 A, when irradiated for not more than 1% of the half-life at a thermal neutron flux of 1013 n 9 cm -2 9 sec-1; this means that the calculated sensitivity exceeds 20 pA/mg. (b) The time of measurement should be less than one week after the start of the irradiation; this means that 3 rain ~< 2 -1 < 1 week, corresponding to halflives of 125 sec < ~ < 4.85 days. These criteria are by their nature somewhat arbitrary, but serve to select elements for which neutron activation followed by ionization chamber measurement compares favourably with other methods of analysis. Elements with a calculated sensitivity of less than 10 -4 pA/mg produce an ionization current of less than 10 -14 A when a quantity of 100 mg is irradiated for one half-life at a thermal neutron flux of 1013n 9 cm -2 9 sec -1, and are not detected by 7-ionization chamber measurement. These elements include H, C, N, O, Si, and P, and are well suited as matrix and container materials in the determination of elements with satisfactory sensitivity. J. Radioanal.
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K. HEYDORN: NATIONAL PHYSICAL LABORATORY IONIZATION CHAMBER
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Table 2 Elements suitable for determination by thermal neutron activation followed by y-ionization chamber measurement, listed in the order of decreasing sensitivity Element
Indium Europium Manganese Gold Dysprosium Vanadium Lanthanum Sodium Gallium Bromine Samarium Iridium Tungsten Arsenic
Radionuclide
Half-life
ll6mlIu 152mlEt1
55.0 9.3 2.58 2.69 139 3.75 40.2 15.0 14.1 35.4 46.7 18 23.9 26.4
5nMn aSSAu t65Dy 52V ~40La e4Na 7~Ga 82Br
1~3Sm ~94Ir a87W
76As
m h h d m m
h h h h h h h h
T h e a b o v e c o n s i d e r a t i o n s are based on i r r a d i a t i o n in a purely t h e r m a l n e u t r o n flux, a n d a c t u a l sensitivities a n d b l a n k values m u s t be d e t e r m i n e d e x p e r i m e n t a l l y f o r each p a r t i c u l a r a p p l i c a t i o n .
Conclusion
T h e use o f a N a t i o n a l Physical L a b o r a t o r y i o n i z a t i o n c h a m b e r t y p e 1383A for d e t e r m i n a t i o n o f arsenic at the m i l l i g r a m level b y n e u t r o n activation analysis was shown to give satisfactory precision a n d accuracy. Arsenic has the lowest calculated sensitivity a m o n g the elements selected as suitable for d e t e r m i n a t i o n b y this m e t h o d , a n d it m a y therefore be c o n c l u d e d t h a t the r e m a i n i n g elements in this g r o u p will also be a m e n a b l e to satisfactory d e t e r m i n a t i o n in a c t u a l practice.
References
1. G. W. LEDDICOTTE, ORNL-1088, 1951, p. 59. 2. J. W. G. DALE, W. E. PERRY, R. F. PULFER, Intern. J. Appl. Radiation Isotopes, 10 (1961) 65. 3. J. W. G. DALE, Intern. J. Appl. Radiation Isotopes, 10 (1961) 72. 4. K. HEYDORN, Intern. J. Appl. Radiation Isotopes, 18 (1967) 479. d. Radioanal.
Chem. 10 (1972)
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K. HEYDORN: NATIONAL PHYSICAL LABORATORY IONIZATION CHAMBER
5. 6. 7. 8. 9. 10.
T. L. ISENHOUR, G. H. MORRISON, Anal. Chem., 36 (1964) 1089. M. FEDOROFF, Nucl. Instr. Methods, 91 (1971) 173. M. OKADA,Anal. Chim. Acta, 24 (1961) 410. F. BAUMG.~RTNER, Tabelle zur Neutronenaktivierung, Karl Thiemig, MiJnehen, 1967. O. T. GOLDMAN, J. R. ROESSER, Chart of the Nuclides. 9th ed., General Electric, 1966. C. M. LEDERER, J. M. HOLLANDER, I. PERLMAN, Table of Isotopes. 6th ed., J o h n Wiley, New York, 1967. 11. K. HEYDORN, Clin. Chim. Acta, 28 (1970) 349. 12. K. HEYDORN, Nuclear Activation Techniques in the Life Sciences, I A E A , Vienna, 1967, p. 179.
J. Radioanal. Chem. 10 (1972)
