I
RALPH H. MULLER University of California Los Alamos Scientific Laboratory, Los Alamos,
N. M.
Interaction of Beta Particles with Matter Beta gaging has saved a great deal of money. It is so important that further extensive research on the theory of action should uncover new industrial applications
0
F THE countless scientific and technical applications of nuclear phenomena perhaps none is receiving greater attention than the production of power. The tremendous activity in our country in this direction is aimed a t gaining knowledge and experience and is not characterized by the urgency that confronts other civilized nations. There is real urgency in the case of Great Britain and France, as the events of the past year have shown. A large part of the economy of these highly industrialized countries can be severely dislocated by the intrigue and nationalistic moves of small underpriviledged countries which have little to offer progress other than obstruction. Consequently, the intensive development of nuclear power for these countries must go beyond their immediate needs for power. For them it is long-term insurance, possibly for national survival. These factors have more to do with the main topic of this discussion than is immediately apparent. I t has often been said that what we can accomplish by the use of radioactive isotopes can be greatly extended in the future, when these materials become still more abundant and cheaper. It is barely possible that we may have to hasten and accelerate studies on more widespread uses of radioisotopes. One day they may reach such abundance that we may have to bury them or be driven to the indignity of trying to find some way of disposing of these lethal riches. Of the manifold applications-tracer studies are well known and applied io practically all branches of science and technology-the manner in which nuclear radiations interact with matter is a continuing challenge. In many respects, application has frequently outrun fundamental knowledge, particularly in the case of beta particles. ’ Of nearly 100 radioisotopes which are now readily available, some 16 are pure beta emitters ranging in half lives from some 2.5 days to 300,000 years.
raise a number of intriguing theoretical questions, the answers to which are not completely clear. No attempt is made here to cover the subject completely; a few representative examples may suffice. They may illustrate once more that “nothing succeeds like success” and it is
Isotope Pml47 TI204
Sr9o Ce144
Ru106
of paper, plastics, rubber sheeting, and metal foil. By incorporating automatic control the thickness can be maintained a t desired levels. I n this method, the absorption of beta particles is measured. Some sources suitable for this purpose are:
Screen, Dimensions, Cm. M~./ Length X Width Sq. Cm. 5-10 X 0.3-1.25 5 2-30 X 0.5-2.5 15
Source Construction A g strip sand., P t face Electrodeposit on Cu; Cu, Cd overdeposit Ag strip sandwich 2.5-30 x 0.3-2.5 A g strip sandwich 2.5-30 X 0.3-2.5 Same a s Ce144
not always necessary to know precisely what is going on, to achieve useful results. Gaging Operations. Some startling economies have been achieved by using beta particles in gaging operations. Elegant machines have been built for automatically recording the thickness
30-50 50
Mc.
Strength Mc./sq. cm.
5-20 80 max.
0.5-1
500 max. 5-20
250 max. 1-6
1 max.
A typical installation is illustrated in Figure 1. Automatic machine control using the output of P-thickness gages as a primary signal will provide an average precision to about 2% of nominal thickness. Improvements effected by using this type of equipment instead of conventional methods include:
Applications of Beta Particle Techniques Most of these applications are eminently practical, yet hardly one does not
Figure 1.
Typical installation for beta-particle gaging VOL. 50, NO. 2
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Figure 2. Hydrogen-carbon gage using beta ray source
ratio
Improvement in
Maintaining
Thickness
System
Fourdrinier paper machine Tandem cold-roll metal mill Rubber sheeting calender Rubber-tire fabric calender Plastics calender Saturator dip coater Abrasive making
4-fold 6.f-fold
3.2-fold 1.9-fold 2.8-fold 2.5-fold 2 .O-fold
Gages of this type, whether manual or automatic, are used routinely for measuring thickness of materials ranging from 1 to 1200 mg. per sq. cm. For best accuracv about half of the betas should be absorbed. For a given beta source materials ranging from 0.2 to 4 times half thickness can be measured. and accuracy to 1% can be obtained between 0.5 and - 2 of half thickness ( I ) . By utilizing the phenomenon of backscattering, the thickness of coatings can be measured. In this case, betas are directed toward the coating and those lvhich are backscattered (roughly sprakCavity for /3 emitter
1
ing, “reflected”) are counted or recorded. For each application optimal conditions must be observed, and these have been worked out. The method presupposes appreciable differences in atomic number between coating and the base. To select a single example: it is possible to control, continuously and automatically, the thickness of zinc coating on 15-mil steel to an accuracy of *0.06 ounce. The true nature of this back-scattering phenomenon is the principal topic of this article. It is an extremely complicated phenomenon, but in no way has it presented great difficulties in such useful and precise applications as just described. Many other useful applications have been made of the absorption of beta particles. Smith and Otvos have described an elegant and precise means for determining the hydrogen content of liquid hydrocarbons (7). The principle is shown schematically in Figure 2. More recently beta techniques have been used in detecting the effluent from a vapor phase chromatographic column (2). The first method has given rise to two commercially available instruments. The absorption of betas from carbon-14 (usually in the form of barium carbonate) has been used to measure the thickness of aluminum foils and plastics and is particularly useful in measuring the thickness of thin sample disks for infrared spectrophotometric analysis. An indirect application uses beta particles from strontium-90 to generatr bremsstrahlen from a lead target to measure and record the density of silt on the bottom of large bodies of water?or the silt content of moving streams, This detector is carried along the bottom from a surface vessel. Details of measuring head are shown in Figure 3. The source is in the limb ~f the plummet a t the left. As the betas from strontium-90 are slowed down by the lead target, bremsstrahlen are gtn-
Figure 4. Typitransmissiontarget source
4 cal
0010 in 0065 In
SCINTILLATION
Figure
3.
crated. These are continuous x-rays. In the passage toward the detecting limb (through water and silt) they strike a scintillating crytal surrounding a photomultiplier tube. The observed counting rate depends upon the degree of absorption by water and silt (I). Other phenomena that arise from beta particle interaction require a brief resume of facts for their description. Beta particles are high speed electrons which arise from the nuclear disintegration. They are not monoenergetic but have all velocities from zero up to a well defined maximum, which in the case of strontium-90 is 0.61 m.e.v. and from the daughter element yttrium-90 is 2.18 m.e.v. The distribution of mergy is precisely defined by the Fermi relationship. The distribution in energy and its maximum value can be measured with high precision with a beta spectrometer. Aside from the complexity of energy distribution. betas interact. with
Collimator may be placed here \ P l a s t i c absorber for scattered
‘
\
betas
\ \
I-
0.097 in
/ Copper-plated face of holderFigure 5. Reflectionsoft solder target source after assembly
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INDUSTRIAL AND ENGINEERING CHEMISTRY
IX -rays
\
Target
-I
Measuring head
\ Magnesium source holder
Shielding
NUCLEAR TECHNOLOGY
r Transmission target leod 0 2amm thick
-
8
50
IC0 Energy
t
Retlection target leod - C 17 m m thick
150
200
250
- Kev
Energy
C
- Kev
Energy
- Kev
Figure 6. Beta-ray excited x-ray spectra: SrgO-YgO betas (0.2 cm. plas\ tic filter)
Figure 7. Spectrum from composite Ba-Pb target: reflection target source, SrgO-YgO betas
Figure 8. Lead fluorescence spectrum: excited by transmission target source, SrgO-YgO betas, uranium primary target
matter in no single or simple fashion. I n passage through matter, they are scattered. Even thin foils diffuse the beam. They are deflected by atomic nuclei and can interact with extranuclear electrons in several ways. The mere process of being slowed down generates an x-ray continuum called bremsstrahlung and this increases with the square of the atomic number and with the energy. Under favorable conditions, characteristic or almost linelike x-rays can be generated, superimposed on the bremsstrahlung. At very low energies, the betas can produce ionization similar to excitation by electrons from thermionic sources. Scattering is so extensive in massive targets that betas can return from the target more or less in the same direction from whence they came. This apparent “reflection” is called backscattering. Of these several phenomena, the generation of radiation has several fascinating aspects (6). It is possible to use beta sources for generating useful amounts of x-rays for radiographic and other purposes. In intensity, they hardly compare with conventional x-ray sources, but a t this early stage no one wants to look the gifthorse in the mouth. Figure 4 shows a transmission type of source, in which the betas traverse the target material. The reflection type shown in Figure 5 is almost identical with a conventional x-ray tube, in which the beta source of electrons replaces the electron gun. Plastic absorbers trap all betas and permit only x-rays to be emitted. T h e characteristic K radiation of lead a t 74 k.e.v. is shown for the two sources in Figure 6. Characteristic
peaks from a composite barium and lead target are shown in Figure 7 . Figure 8 results from bombardment of a uranium target, which then excites fluorescence emission in lead. Some typical radiographs are shown in Figure 9. Although the exposure is long, the source represents some $15 worth of strontium-90 at present prices, A thickness or density measuring system is illustrated in Figure 10. The primary purpose of such investigation is to provide x-rays from beta sources a t relatively low energies-of the order of kilovolts. Competitive gamma sources are mostly of far higher energy and not so readily absorbed by matter.
Conversely, conditions may be so arranged that the stopping of the betas produces primarily the bremsstrahlen or x-ray continuum. These have many uses, only one typical application of which has been mentioned.
Conversion of Beta Particle Energy into Electricity One other phenomenon is receiving considerable attention a t the present time-direct conversion of beta particle energy into electricity. All atomic reactors intended for power production produce energy in the form of heat, and
Figure 9. Typical radiographs taken with bremsstrahlung source: 10 mc, 1 1 inches, 34 hours
Figure 10. system
Thickness or density measurement
Servo Amplifier
Compensating absorption wedae
I
, Servo motor
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IL
w (iL
10
Figure 11.
20
30
40
50
EO
70
EO
Backscattering as function of atomic number ATOMIC NUMBER
this is utilized in the conventional manner to drive steam turbines. The possibility of direct conversion to electrical energy is challenging, but a t present it can be done only on a very small scale and a t very low efficiencies. The first “atomic batterv” was demonstrated in 1913 by Moseley. By collecting the beta particles from a source in a n evacuated vessel he produced a potential of 150,000 volts. The total charge accummulated by the system was, of course. extremely small. I n recent years several phenomena have been employed to increase the total available electrical energy. Direct collection of beta particles, in the Moseley sense, has been improved. Another scheme uses the contact potential between dissimilar metals or compounds. in which the space between the electrodes is highly ionized by a beta emitter. Multicellular systems based upon this principle have provided useful batteries. Another system depends upon dielectric storage. Bombardment of the transistorlike junctions also provides a source of electrical energy. Multijunction thermocouple systems have been used for direct conversion of heat into electrical energy. Several varieties of atomic batteries are in commercial production and they have very definite uses, such as charging electrometers and furnishing high potentials under negligible current drain. Potentials of the order of kilovolts and currents of the order of 1O-I2 ampere are representative of such systems. In the case of junction devices, potentials of the order of 1 volt and currents in the microampere range are feasible. Another advantage is the long life of such systems. Phosphors, The very old principle of scintillation, as exemplified by luminous watch dials, has been vastly improved with the advent of abundant isotopes. Fairly brilliant light sources are now available in which beta emitters are incorporated in phosphors. Adequate shielding and protection are accomplished by optimum thickness of plastic. The U. S. Radium Corp. manufactures a large variety of such light sources. A combination using promethium-147 is used for railway-yard sig‘
208
Figure 12. Backscattering of beta particles as discontinuous: function of Z, bur linear with Z in each period
nals. The decay and useful life of such luminous sources can be inferred from the half life of the beta source, although in the case of some very long-lived emitters such as carbon-14 (5568 years) there is some question about the useful life of the phosphor, which may exhibit slow destruction under continuous bombardmen t , Eminently practical applications of beta bombardment have been forthcoming. Almost all raise fundamental questions which are extremely interesting. Nature of Backscattering
For more than 4 years the backscattering of beta particles by matter has been studied, primarily to gain precise information on how the relative backscattering depends upon the nature of the target. Although a complete description of these studies has been published (4,5),the main conclusions are presented here to show how these results might applv to industrial problems. The earlier data, and a disappointing proportion of current data, pretend to describe backscattering as a function of atomic number of the scatterer somewhat in the manner in Figure 11. For such data, the relative backscattering is roughly proportional to the cube root of Z and some empirical exponential expressions can also be fitted to the data. There is a disarming simplicity in almost every phenomenon, if the information is sufficiently meager. Upon measuring some 36 elements and several hundred compounds, it was found that no simple continuous function will relate backscattering to atomic number. T h e actual state of affairs is shown schematically in Figure 12, in which backscate srrictly linear in Z the periodic system, with abrupt inflections a t atomic numbers 10, 18, 36, and 54 corresponding to the rare-gas configurations neon, argon, krypton, and xenon. T h a t these are the true inflections was proved by
INDUSTRIAL AND ENGINEERING CHEMISTRY
~
synthesizing pur? single crystals of sodium fluoride, potassium chloride, rubidium bromide, and cesium iodide. I t can be shown that these substances duplicate the atomic numbers of IO, 18, 36, and 54 to a very close approximation. T h e equations representing the linear behavior in each period have been determined with high precision. For all compounds an effective atomic + number, 2,is calculated. For a compound ABC
z= + ZAfA
ZEfB
$. Z C f C , etc.
where the f ’ s are the weight fraction of each element and the 2 ’ s are the respective atomic numbers. O n this basis a crystal of calcite (CaC03) has an effective atomic number 2 = 12.563 and its relative backscattering should follow the linear equation for Period 3, being intermediate between magnesium (12) and aluminum (13). Such interpolation can be checked to within 0.08
70.
T h e precision of these results can be inferred from another example shown in Figure 13. Here, if we wished to regard crystals of sodium chloride and potassium chloride as cardinal calibrating points, they would predict, within some 0.07%, the scattering of calcite which contains no kinds of atoms to be found in the two other crystals. Out of some dozen other conclusions, a few interesting facts may be mentioned. Isomers exhibit identical backscattering of beta particles but fortunately tliev differ in absorption and almost rxacrly in the ratio of their densities. There is no measurable difference in backscattering from a substance in the liquid or solid state despite appreciable density differences. This is not true in absorption of betas; indeed, careful density corrections are necessary if the temperature varies. Obviously, this does not apply to the gaseous state, in which the scattering is very small. As shown in Figure 14, the backscattering can be measured from solutions.
NUCLEAR TECHNOLOGY BACKSCATTERING BY ALKALI HALIDE SOLUTIONS 0
,__I___
V U E 5 FOR SYNTHETIC CRlsTAL
csci
/
KCL
NaCl LiCl 1
\
12
13
I 14
1
15
I 16
1
I
17
18
ATOMIC NUMBER
Figure 13.
Precision of results WEIGHT FRACTION O r SOLUTE
The relationship between backscattering and weight fraction of solute, at least in the case of the alkali chlorides, is linear, permitting the extrapolation to pure solid. This is not necessarily true for all solutions; there is definite evidence for nonlinearity, especially for systems that deviate markedly from Raoult’s law. Organic compounds obey all these regularities, except that hydrogen exhibits an apparent “negative backscattering,” presumably due to absorption. Fortunately, the hydrogen defect is exactly proportional to the weight fraction of hydrogen in the compound. Muller has evaluated this defect by precise measurements on four solid hydrocarbons ( 3 ) . These general relationships enable one to confirm the nature of any element or any compound in a few minutes. At the present time the converse is not true; one cannot define the composition uniquely if its constituent atoms are unknown. There is increasing evidence that this limitation will not hold very long. Differences in angular distribution and energy distribution and other related information, now being studied, may remove these limitations. These studies have been confined to samples of “infinite” thickness. The exact dependence upon thickness, although probably worked out for a few systems, is not generally known. One still encounters vague rules and approximations relating saturation backscattering to thickness and as fractions or multiples of infinite thickness for transmission. A few precise measurements indicate that the backscattering can be expressed precisely as a function of thickness, but these results are confined to a limited number of elements. These general conclusions are very useful and can be verified with high precision, but many subsidiary effects must be taken into account before the phenomenon of backscattering can be
Figure
14. Backscattering by alkali halide solution values
of synthetic crystal described in a coherent and unified form. This is rapidly becoming possible,‘ as the contributions of bremsstrahlung and characteristic radiation are systematically studied. For example, most recently a careful study of the excitation of characteristic x-rays has been made by bombarding targets with beta particles. T h e important technical applications of this phenomenon were mentioned by Reiffel and Humphreys (6), and several are the basis of patents. The X lines of strontium, molybdenum, silver, cadmium, tin, antimony, iodine, barium, tantalum, tungsten, platinum, mercury, lead, and bismuth have been excited. Precise measurements with a 100-channe1 pulse height analyzer permit the determination of energy to within 190 volts. For a single element in the target it is impossible to mistake its identity. In the case of tantalum (2 = 73) the peak can be located with a n error about ’/I* of the difference between the nearest adjoining elements (Hf, 2 = 72 and W, Z = 74). T h e x-ray peaks which are okserved correspond to the Xal line of the target. I t is certain that the peak also contains Kaz but it is not resolved. Some indication of its presence was obtained by recording the K line of silver with and without a 2-mil palladium filter. There is a definite narrowing of peak when the palladium filter is used. An interesting commentary on the precision of these methods relates to the palladium filter experiment. From the known mass absorption coefficient of palladium, the expected absorption of the silver radiation could be calculated. T h e observed value was 2y0 low. Recalculation showed that the 2-mil filter should have been 50 microinches thicker! The principal object of this discussion
has been to show that such eminently practical and important applications as beta gaging involve phenomena which. for their own sake, are worthy of further: extensive research. I t is not to be inferred that present industrial practice is hopelessly empirical. Quite the contrary. These elegant machines embody the finest instrumental techniques and for the particular problem a t hand, the optimum conditions have been worked out with great care. Their successful operation does not require a completely rigorous knowledge of beta particle interaction. Whatever is learned in the future about the many aspects of beta particle interaction, it will not invalidate current practice. One can only hope that it will lead to new and unexpected applications. literature Cited (1) Crompton, C. E., Proc. Intern. Conf. on Peaceful Uses of Atomic Energy. Vol. 15, p. 131, United Nations, New York, 1956. (2) Deal. C. H.. Otvos. J. W.. Smith. V.’ N., Zucco, P.’ S., Anal. Chem. 28, 1958 (1956). ( 3 ) Muller, D. C., Zbid.,29, 975 (1957). (4) Muller, R. H., Zbid., 29,969 (1957). Rev. 93, 891 Muller, R. H.,. Phvs. . (5) (1954). ( 6 ) Reiffel, L., Humphreys, R. F., Proc. Int. Conf. on Peaceful Uses of Atomic Energy, Vol. 15, p. 291, United Nations, New York, 1956. (7) Smith, V. N., Otvos, J. W., Anal. Chem. 26, 359 (1954). RECEIVED for review April 3, 1957 ACCEPTED December 9, 1957 Division of Industrial and Engineering Chemistry, Symposium on Nuclear Technology in the Chemical and Petroleum Industries, 131st Meeting, ACS, Miami, Fla., April 1957. Work performed under auspices of the U. S. Atomic Energy Commission. \
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