DATA HANDBOOK FOR SENSITIVITY CALCULATIONS IN NEUTRON ACTIVATION ANALYSIS

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EUROPEAN ATOMIC ENERGY COMMUNITY - EURATOM
DATA HANDBOOK FOR SENSITIVITY CALCULATIONS
IN NEUTRON ACTIVATION ANALYSIS
by
F. GIRARDI, G. GUZZI and J. PAULY
1965
Joint Nuclear Research Center
Ispra Establishment — Italy
Chemistry Department
Nuclear Chemistry Service CONTENTS
1 — INTRODUCTION 5
2 — THEORY 7
3 — MEASUREMENT OF THE ACTIVITY 9
4 _ USE OF THE HANDBOOK 11
4.1 — Table X
4.2 — Tables X"»-a and X"»-o2
5 — EXAMPLES3
6 — BIBLIOGRAPHY6
7 — TABLES 19 DATA HANDBOOK FOR SENSITIVITY CALCULATIONS IN NEUTRON ACTIVATION ANALYSIS
1 — INTRODUCTION activation analysis or not, and have a rough idea of « how » it can be
resolved, without the necessity of doing sophisticated calculations. All
One of the principal advantages of activation analysis is the the data which are necessary to calculate the sensitivity that can be
possibility of changing the sensitivity and selectivity of the analysis obtained in different experimental conditions are assembled for most
by varying the experimental conditions. The counting rate of a radio­ elements in a form which in our hope will make the calculations simpler,
isotope formed by neutron activation can be changed by a factor of 10fi
so that also trained activation analysts could find this Handbook
or more by changing experimental parameters such as irradiation time, useful in the day-to-day routine.
position in the realtor, nature of the counting equipment, counting
We tried to keep the experimental conditions for which the
geometry, use of discriminators or absorbers, etc. As the variation of
sensitivity limits Avere calculated as close as possible to the real working
the counting rate obtained is different for different radioisotopes, it is
conditions for practical analysis. A few limitations were nevertheless
possible, with a convenient choice of the experimental conditions, to
necessary :
increase the counting rate of certain radioisotopes and lower it for
others, in order to favorise the solution of an analytical problem. A 1) The irradiations are assumed to be done in a pure thermal
careful optimization of the experimental parameters can then simplify neutron flux. Only reactions produced by these particles are considered
the problem of the activity measurement and avoid, in many cases, the and thermal activation-cross-sections used in the calculations. This
use of chemical separations. assumption holds fairly well for irradiations done in heavy water
reactors, which generally have a small epithermal component in the This characteristic of activation analysis is very advantageous for
neutron flux. Scientists who only have available swimming-pool reactors many applications, but it brings in the necessity of doing preliminary
or graphite reactors will find experimental counting rates of the elements calculations, if the operator wants to know whether a certain problem
having a high resonance integral (Ta, U, Sb, In, I, Au, ...) greater than
can be resolved or not and which are the best conditions to resolve it.
those reported.
These calculations are often tiresome, and scientists might be
discouraged in their first approach to activation analysis. 2) Most of the data presented refer to measurements of activity
This Handbook is mainly devoted to those who want to know done with a gamma-ray spectrometer (photopeak counting rate). A
rapidly whether a certain analytical problem can be resolved by 3x3 inch Nal(Tl) crystal in an end-on geometry (Fig. 1) was used as gamma-ray detector. For some elements producing by activation only
pure beta emitting radioisotopes, the calculations of the sensitivity
refer to a front-window Geiger-Muller counter. The choice of the
measurement conditions which are those most frequently used in our
laboratory is of course somewhat arbitrary, although the working
conditions of many activation analysis laboratories are similar to those
described.
The main reason which has convinced us to present sensitivities
corresponding to counting rate3 really obtained under certain practical
conditions instead of the more general « disintegrations per minute »
is that the passage from the disintegrations per minute theoretically
calculated to the practical counting rates, cannot be done simply.
The assumption that the minimum detectable activity corresponds ^^^¿^^^^^^^^
for all elements to a same number of disintegrations per minute
is often made to simplify the calculations. This procedure can however
frequently induce large errors. We have therefore preferred to calculate
the minimum detectable activity separately for each radioisotope in
certain well defined conditions.
3) A choice between different (η, γ) reactions on the same element
was done to define « analytical reactions » and a choice of different
peaks present in a gamma spectrum was also done to pick-up « ana­
lytical peaks ». This choice does not necessarily mean that other
reactions or other peaks cannot be used for activation analysis. It
simply indicates that in normal problems those reactions and those
peaks are the ones which are more likely to be chosen for analysis. The
complex decay curve of some of the peaks in the gamma spectra due
to the formation of both long-living and short-living radioisotopes from
the natural isotopie mixture, was also brought into evidence, to make
Fig. 1 — Source-detector geometry of the gamma-ray spectrometer. 1 : source
people aware of possible complications, when decay factors are
container — 2 : radioactive source — 3 : lucite beta absorber — 4 : detector mounting
calculated. — 5 : Nal(Tl) detector. Thicknesses : d = 2 mm, e = 4 mm, ƒ = 6 mm. generally reported in the tables of nuclear constants, by the 4) The sensitivities were calculated by using nuclear constants
reported in the literature. When some of the data were lacking or equation :
σθΝ values reported by different authors were grossly discordant the
(2)
M sensitivity was determined by irradiating weighed samples of the
element in a flux of thermal neutrons (with an epithermal component where 0 is the natural isotopie abundance of the target element,
lower than 1 °/0) and measuring experimentally the activation rate. M its atomic weight and Ν the Avogadro number.
The precision that can be expected on the values of sensitivity limits E is the counting efficiency of the detector i.e. the ratio between the
reported depends on how well the nuclear constants are known and on counting rate obtained and the emission rate of the counting
how similar to ours are the irradiation and counting facilities available.
source for the radiation used.
On the first point it must be noted that a comparison of theoretical
a is the absolute abundance of the radiation measured.
(using literature data on nuclear constants) and experimental activities
S or « saturation factor » is a function of the irradiation time Τ : done on 13 elements among the most frequently determined showed
an agreement within 9 % for 11 elements while the other two were 0.693T
within 20 % (7). On the second point a scientist without a particular S = 1 — e t'A (3)
knowledge in activation analysis can only consider the sensitivity limits
where t% is the half­life of the radioisotope formed.
indicated as limits that can be obtained with a certain set of experimental
D or « decay factor » is a function of the decay time t conditions described in the following pages, and which is attainable
without particular difficulties ; but it is also possible to work out 0,693t
D = e ty* (4) correction factors to take into account differences between their
experimental parameters and ours. Equations 3 and 4 are holdingwhenthe radioisotope is formed
exclusively by neutron captureofastableelement. In many cases
radioisotopes which can be usefullyemployedin activation analysis are 2 — THEORY
also formed by the decay of other radioisotopes. The laws of saturation
The activity A in counts per second, of a radioisotope formed and decay become in these cases more complex (8) and will not be
by neutron activation is related to the weight of the irradiated element reported here. They have been taken into account in the saturation
W by the equation : and decay curves reported.
We can define the specific activity A