2.2 Physical-metrological fundamentals of constructing the RUTS systemsthe RUTS systems
2.2.5 System of reproduction of physical quantity units
2.2.5.3 System of unit reproduction in the field of ionizing radiation parameter
The system of unit reproduction in the field ofionizing radiation parameter measure- ments(IRPM) includes measurements (widely used in science and practice) of the ra- dionuclide activity, dosimetry of photon, beta- and neutron radiations, measurements of the field parameters and sources of electron, photon and neutron radiations. The RUTS system in the field of ionizing radiation parameter measurements is based on seven national primary measurement standards and seven special national measure- ment standards. Usually the measurement standard consists of a number of setups.
Each of these setups reproduces the dimension of one and the same unit, but within different ranges of particles or photons or for different kinds of radiation, etc.
An analysis of the existent system of measurement standards reveals a number of significant drawbacks caused by the lack of a unified approach in the formation of the RUTS system in such a complicated field, characterized by the diversity of dif- ferent kinds of radiation (and the corresponding radiation sources), as well as condi- tions of their measurements (energetic ranges, environment, specificity of radiation (bremsstrahlung, pulse radiation, etc.)). Only one RUTS system designed for mea- suring the activity of various kinds of radiations and various RUTS systems used in dosimetry (photon, beta- and neutron radiation) functions in such a manner.
The special standard of the unit of neutron flux density has a lesser error than the primary standard of the same unit (at least on a board of the range of reproduction).
Section 2.2 Physical-metrological fundamentals of constructing the RUTS systems 153 Only one special national measurement standard functions for reproducing the unit of volume activity. There are some other examples of drawbacks.
A system approach to constructing a system of unit reproduction in a given field of measurements has to be based on the general physical ideas of both the ionizing radiations and the field and its sources, the choice of initial (base) quantities in terms of which of the remaining quantities (completeness of the system description, interre- lation of elements, hierarchical dependence) can be expressed. An attempt of this kind was made in 1974 by V. V. Skotnikov et al. [445].
Let us try to systemize and develop the ideas of this work in the light of a mod- ern overview. First, we should defined more exactly physical objects and phenomena participating in the process of measurement in this field:
proper “ionizing radiation”, i.e., all radiation types connected with the radioactive decay of nuclei (˛-, ˇ-, - and conversion radiation) or with nuclear reactions (fluxes of charged and neutral elementary particles and photons), as well as X- radiation and bremsstrahlung (i.e., hard electromagnetic) radiation caused by the interaction of radiation of the types indicated with the atomic systems;
the sources of this radiation (mainly the sources of radioactive radiation);
the environment (of substance) with which the radiation interacts;
the phenomena taking place in this environment as a result of radiation passing through it.
By the ionizing radiation (IR) we mean, strictly speaking, any electromagnetic or cor- puscular radiation that is able in an appreciable manner to produce an ionization of environmental atoms through which it penetrates (“in an appreciable manner” means here that ionization is one of the main phenomena when the given radiation passes through the environment).
Since ionization requires a definite energy consumption for overcoming the forces of attracting electrons in atoms (or in molecules), it is possible to say, in other words, that the ionizing radiation is an electromagnetic or corpuscular radiation for which ionizing losses of energy are the main losses when this radiation passes through the substance.
In fact this field includes measurements of neutron flux parameters, i.e., of neutral (in the sense of a charge) radiation, the ionizing action of which is negligible as com- pared to the types of its interaction (nuclear reactions of various kinds). Moreover, the ionizing radiation measurements cover not only parameter measurements of the ionizing radiation itself, but measurements of the sources of this radiation, as well as the phenomena caused by this radiation in the environment.
In a wider plan it may be necessary to single out a field of “nuclear measurements”
covering the whole specific field of measurements in nuclear physics and nuclear en- gineering (including its application fields).
Let the generalized field of radiation [445] be determined as a state in the space, the physical properties of which in a point given and at a given time moment are caused by the availability of particles or quanta at this point. The field of a definite type of particle in a given point and at a given moment in time are characterized by pulse values and energy distributions. Consequently, the main characteristic of the field of radiation has to be a value differential for all the properties of the generalized field.
The most complete and universal (both from a theoretical and practical point of view) characteristic of the fields and sources of IR is the spectral characteristic of radiation or (in a wide sense) spectrum of radiation.
At the given (chosen) space and time coordinates for the system the radiation spec- trum in the quite general form is described by the dependence of the form
S DX
i
ni.rE,t,E,/E Df .rE,t,E,,E i /, (2.2.52)
whereni is the number of particles of a given radiation sorti in the point with the coordinatesrEhaving the energyEand moving in the directionE (unit angular vec- tor) at the moment in timet. At fixed values of the arguments the physical quantity n, corresponding to dependence (2.2.52), has the sense of a differential (space–time, energetic-angular) density of radiation.
The knowledge of this quantity at the points in space interest gives practically every- thing needed from the point of view of measurement quantities and parameters of fields and the sources of ionizing radiations obtained (in most cases with a simple mathematic integration).
(1) Differential radiation flux density'(the number of particles at the point with co- ordinatesrE, having an energyE, which move in a directionE and cross the plane surface of the 1 cm2 area at a right angle to the vectorE for 1 s at the time mo- mentt):
'i.Er,E,,E t /Dni.rE,E,, ,E t /Ui, (2.2.53) whereU is the movement speed of particles under the same conditions.
(2) Differential radiation flux through the planeS:
Pi.Er,E,,E t /D
Z sCs
sD0
'i.rE,E,,E t /dS. (2.2.54)
(3) Differential intensity(energy flux density) of radiation:
Ii.rE,,E t /D Z 1
ED0
'i.Er,E,,E t /EidE. (2.2.55)
Section 2.2 Physical-metrological fundamentals of constructing the RUTS systems 155 (4) Differential radiation absorbed dosefor the given substance (B) characterized by the dependencei.Ei/of the absorption cross-section of theitype on the energy:
D.Er,b/D d"N
d m; (2.2.56)
d"ND Z
t
X
t
R
E
R
'i.Er,E,,E t /EidEd R
E
R
'i.rE,E,,E t /dEd D Z
t
X
t
R4
D0Ii.Er,,E t /d 'i.rE,t / . (5) Activity of radionuclides:
A.t /D dN
dt D 1 Pi, E0
Z 4
D0
Pi.rE0,E,,E t / d jr0D0,EDE0, (2.2.57) wherePi,E0are the portion (part) of the given type of radiation (i /with the energy E0for one decay act.
(6) Source outputwith respect to the radiationi: Wi.t /D4 r2
Z 1
ED0
Z 4 D0
'i.rE,E,,E t /dEd , (2.2.58) etc.
The relationships indicated above express the idealized (irrelative of any particular real conditions) connection between the corresponding quantities, arising from their definitions.
In each real case it is necessary to take into account the specific conditions of getting information about the input parameters and the quantities of these equations, and to use in a number of cases additional regularities and parameters.
Problems of such a type arise in the process of constructing a particular measuring setup (or method). However, the indicated connections and subordination among var- ious quantities should not be neglected, since they are principal (it follows from the definitions of physical quantities).
It should be noted that in a number of cases the relationships indicated include the parameters (cross-section of reactions, average energy of ion formation, parameters of the decay scheme), which are a product of research experiment, and, therefore, here the importance of reliable reference data in the field of nuclear physics is clearly seen.
Thus, within the framework of the system under consideration in the capacity of initial physical quantities it is possible to adopt the quantities used as the arguments in initial spectral dependence (2.2.52):
n,Er,t,E,.E (2.2.59)
From the point of view of measurement accuracy, only two problems specific for the given field of the quantity measured are problematic, i.e.,n(a number of particles)
andE(an energy of particles), sincerE,t, and, being the space–time characteristics,E play the role of classical (rather than quantum-mechanical) quantities in all real cases and, consequently, their measurements relate to the classical field of measurements.
The problem of measurement (a number of units and photons) is reduced, as a matter of fact, to the problem of determining the efficiency of radiation detectors and record- ing system. Let us note that the second part of this problem is common for all kinds of radiation and mainly depends onn. But at the same time the first part (a detector) strictly depends on both a sort (a kind) of radiation (i) and its energy range.
However the solution of this main problem, i.e., the determination of the detector efficiency(the detector sensitivity to various kinds of radiation), may be realized by the way of using radiation sources certified with respect to the activity:
"i D .ni/det
t .Ai/source4 , (2.2.60)
where"i is the detector efficiency with regard to radiationi,.ni/detis the speed of counting particles,.Ai/sourceis the source activity certified with regard to radiationi, andis the collimation solid angle.
At the same time the measurand is only the solid angle and speed of counting in the detector (such as it is, i.e., it does not coincide with the true speed of counting in the given place of the radiation field).
It is quite possible that in the future the activity of sources will be determined on the basis of decay constants and radioactive source mass.
The radionuclide activity is unambiguously connected with its decay constant (N) and amount of radionuclide atoms (N) in a source (preparation) in the form
ADN, (2.2.61)
where D 2 ln 2T is the decay constant characterizing the radionuclide given and de- termined by a nuclear structure of an isotope. It can be determined at a rather high accuracy (error 104/[317] for some isotopes with a convenient half-life period T/2 by means of relative measurements of a number of ejecting particles in the certain time intervals.
The determination of the number of radionuclide atoms N in a source is a much more complicated problem. However, taking into account recent successes in prepar- ing pure substances (including those that are applied in radiochemistry and mass- spectrometry) as well as in analyzing their spectral distribution and determining the Avogadro constant, it seems possible that this problem will be solved in the very near future.
The problem is reduced to obtaining a sufficiently pure sample of a monoisotope (with mixtures0.01 %), its weighing (even among existing standards in the field of ionizing radiations the radium mass measurement standard has the best accuracy of reproduction and maintenance (0.1 %)), recalculating the mass into a number of
Section 2.2 Physical-metrological fundamentals of constructing the RUTS systems 157 atoms with the help of the Avogadro constant and atoms of the isotope mass (M):
N D mN0
M . (2.2.62)
If one succeeds in preparing a sample (a source) of some isotope with a convenient T/2 and in measuringNin it at an error of0.1 %, then it will be possible to determine its activity and, consequently, the efficiency of the detectors for some kinds of radiation at the same error.
It should be noted that in both the existing method of determining the activityA and the method suggested for finding the detector efficiency, the necessary element of a priori knowledge is the sufficiently precise knowledge of the radionuclide decay scheme, i.e., knowledge of decay channels, relationships of their intensities, energies of radiation in each channel, and other elements of the decay scheme.
In other words, the determining factor in the development of this field of measure- ments at any setting is the advanced development of nuclear–spectrometry methods and means of measurements (at least it concerns the nuclear spectroscopy in relative measurements).
The problem of measurement of another quantity, specific for the nuclear physics, the energyEi, has up to now been mainly solved “inside” nuclear spectroscopy itself predominantly by the method of relative measurements. Only in works, not numerous, which were done abroad, infrequent attempts were made to link measures of the radi- ation energy (mainly of the photon radiation), used in the nuclear spectroscopy, with measures of length in the field of X-ray spectrum (which until recently has not been connected with the field of optical radiation) or with the annihilation mass (i.e., with such fundamental constants as the rest mass of an electron,
m˜e, and light speed C˜e).
Meanwhile, it is known that not long ago a developed optical – X-ray interferome- ter – allows energy scales of optical and X-ray ranges of the electromagnetic radiation to be linked, and high-precision crystal-diffraction spectrometers existing for a long time allow X-ray and-radiation scales to be linked at an accuracy of106.
Thus, the possibility of setting a dimension of the energy unit of nuclear mono- energetic radiation through a dimension of one of the base units of mechanical quanti- ties, in particular the meter reproduced on the basis of wavelength of optical radiation, becomes a real one. At the same time, the energy reference points in the field of X-ray (or-) radiation become the primary ones with regard to the energy reference points in other kinds of nuclear radiation, since even if they are determined with absolute meth- ods, but the determination accuracy nontheless being much worse than that expected for electromagnetic radiation.
Among charged particles there are methods of measuring energies only for electrons .e/, the accuracy of which can be compared to the accuracy of the methods used forN photons.
At the same time three ways are possible for them:
(1) determination of conversion lines energies onE and bonding strength of elec- trons in an atom (i.e., again on X-ray data):
Ee DEEbond;
(2) measurement of electron energies on an potential difference:
EDeV ;
(3) measurement of conversion lines energies in an uniform magnetic field:
EDf .H/,
whereH is the magnetic field strength andis the curvature radius.
Two last methods can be used for controlling the first method (an additional coordi- nation of the measurement units in various fields).
As to other kinds of nuclear radiations (protons, neutrons, and others), the require- ments of practice regarding the accuracy of measuring their energies are less strict and the methods are “painted over” by knowledge of the energies of either photon or electron radiation.
The considered system of physical quantities in the field of ionizing radiations (or, more precisely, of the nuclear ones) as well as the analysis of this system from the point of view of measurements (above all of precision measurements) allow the approach to formation of a corresponding system of interconnected measurement standards in the considered field of measurements to be suggested.
In accordance with this system, in the field under consideration the primary stan- dards of the activity and nuclear radiation energy units have to be the base ones. On the basis of these measurement standards it is possible to develop a standard of an initial differential characteristic of fields and sources of nuclear radiations, i.e., the radiation density.