US2928944A

US2928944A - Apparatus for x-ray fluorescence analysis

Info

Publication number: US2928944A
Application number: US447767A
Authority: US
Inventors: Reiffel Leonard
Original assignee: Research Corp
Current assignee: Research Corp
Priority date: 1954-08-04
Filing date: 1954-08-04
Publication date: 1960-03-15
Anticipated expiration: 1977-03-15

Description

Mafch 15, 1960 REIFFEL APPARATUS FOR X-RAY FLUORESCENCE ANALYSIS Filed Aug. 4. 1954 L EUJZHFQ flEZffEL burl APPARATUS FGR X-RAY FLUORESCENCE ANALYSIS Leonard Reilfel, tChicago, 5., assignor, by mesne assigrn ments, to Research Corporation, New York, N.Y., a corporation of New York Application August 4, 1954, Serial No. 447,767

a Claim. (Cl. 250- 53 This invention relates to a method and apparatusfor X-ray fluorescence analysis in which the characteristics of a sample is determined by analysis of X-rays generated within the sample.

A type of X-ray designated in the art as the K X-rays or characteristic rays have a substantially-fixed energy and frequency for each of the elements. Such X-rays are generated when an atom is excited from an external source of energy to an extent SllffiClEHt to remove an electron from the electron ring closest to the nucleus of the atom, such ring beingreferred to in the art as the K ring. When such an electron is removed, it may be replaced by an electron from the next ring outside the K ring, which ring is designated as the L ring. This transfer of an electron from the L'ring to the'K ring will release or generate a K X-ray.

In some cases the electron removed from the K ring will be replaced by an electron from a ring outside the 1. ring, out the frequency of such an occurrence will be relatively small with respect to the frequency in which the electron removed from the K ring is replaced by an electron from the Lring, so that the Krays will be the principal rays of fixed frequency and energy. In addition to the K rays and similar rays discussed above,

there may be ,a continuous spectrum of Xerays.

The fact that such K rays of fixed frequency are generated has been heretofore utilized-for analysis pur-- poses. In the prior art method, ,X-rays from a sealed-off X-ray tube bombard a sample, such as a solid block or compacted powder, and undergo photoelectric absorption therein. As the atoms involved in the interaction readjust their electronic structure, 'K X-rays are emitted :which have frequencies characteristic of the atoms in the sample. The resulting radiation from the sample is analyzed by diffraction from either fiat or curved crystals and detected by films or Geiger counters. While this prior art method is satisfactory for some purposes, it has certain limitations. In particular, because of the poor geometry attendant to crystal analysis and the extremely low efficiency of X ray production in the X-ray tubeitself, a major limitation has been the availableelectron beam energy in the X-ray tube. Also, the apparatus required for producting the X-rays with the X-ray tube is very bulky, complex and expensive to manufacture and maintain, and utilizes dangerously high potentials. In addition, the crystals can be dificult to prepare and use.

This invention was evolved with the object of overcoming the limitations anddisadvantages-of the prior art methods of X-ray fluorescence analysis and of providing an X'ray analysis method and apparatus which is inexpensive, efiicient and very versatile.

According to an important feature of this invention,

2,928,944 Fatented Mar. 15,1960

analysis in many applications where the use of'the prior art methods is impossible or highly impractical.

In particular, the radio active material maybe disposed in such relation to the sample to be analyzed that energy from the radio active material generates X-rays in the sample. The X-rays generated by the sampleimay then be analyzed and detected by diffraction from crystals and detected by films or Geiger counters, but preferably other analysis and detection means may be utilized, as will be described hereinafter.

In one preferred embodiment of the'i'nvention, the radio active material emits Bet-arays (or electrons) and the materialis disposed in such relationto the sample that the Beta rays directly from the material impinge "on the sample and generate X-rays therein. 'With'this arrangement, an extremely. high energy conversion efficiency is achieved.

In another preferred arrangement of this invention,

the radio active material emits Beta rays which are .filed copending application entitled X-Ray Source,

United .States Serial No. 370,775, new Patent No. 2,797,333, filed July 24, 1953, of which this application is a continuation-in-part, the disclosure of such application being incorporated herein by reference.

According to another important feature of the invention, the energy levels of X-rays generatedby the sample are directly measured. As indicated heretofore, the energy, as well as the frequency, of the characteristic or K X-ray for each element is fixed, and it has been found that the characteristics of the sample may be determined by measuring the level of the energy of the characteristic or K X-rays, instead of'by measuring the frequency of the K X-rays as is accomplished through diffraction from crystals in. the prior art methods. With this direct measurement of energy levels, a much greater efficiency is achieved since there is no energy lost such as encounterel with the diffraction crystals. Also, with this direct measurement, the apparatus can be much more compact and portable, is very simple and it is economically manufacturable. Further, this method can be readily applied to relatively light elements Where crystal analysis is either impossible or-highly impractical.

This direct measurement of energy levels may be accomplished by counters referred to in the art as proportional counters. One preferred type of proportional counter usable in the practice of this invention may comprise a hollow tube or the like filled with a suitable gas; When an X-ray strikes a molecule of such gas, an electron or electrons may be released from such molecule and may strike other molecules to generate further electrons. Electrodes may be disposed in the gas, one of which may be the tube'itself and a source of uni-directional voltage is applied between such electrodes. The electrons released from the molecules by the X-ray may flow through the positive electrode tothe source of voltage, and this current may be measured. With a voltage up to a certain value, this current flow will be proportioned to the energy of the X-ray. This current flow may, of course, be amplified and detected by any suitable means.

Another type of proportional counter usable in the practice of this invention is known inthe art as ascintillatfon counter. This counter may comprise a target of fluorescentmaterial on which .X-rays may be impinged,

The X-rays will generate a certain quantity of light energy which may be detected by a photo-multiplier tube, the output of the photo-multiplier tube being amplified and detected by any suitable means.

It will be understood, of course, that any other type of proportional counter adapted to measure accurately the energy levels of X-rays may be used. Film may, for example, be used.

According to a further feature of the invention, a critical absorber may be disposed between the sample and the detector. Such an absorber may have a characteristic such that the absorption of X-rays gradually decreases as the energy level of the X-rays is increased until a certain energy level is reached at which the absorption is sharply and greatly increased. With energy levels higher than such certain energy level, the absorption may gradually decrease. The energy level at which the absorption is greatly increased is dependent, of course, on the material forming the absorber.

This absorber may be used in a variety of ways for analysis purposes. As one example, if it is desired to determine whether a sample contains a certain element, an absorber may be selected which has a peak absorption at the energy level of the characteristic or K X-rays of such element. Then the response with such absorber may be compared to the response without such absorber to determine the presence of such element.

It will be appreciated that the fluorescence analysis methods and apparatus of this invention may be utilized to determine the elements present in a given sample, whether solid, liquid or gaseous. For example, it has been applied to the measurement of the lead content of gasolines and the detection of metallic contaminants in pharmaceuticals. It may also be used in the monitoring of metallic fumes in air by absorption on filter papers and subsequent continuous analysis. The methods and apparatus of this invention have also been used to determine plating thickness as, for example, silver on brass or nickel on steel. It may also be used to detect pinholes in coatings.

In general, the energy levels of the Beta rays or X- rays used to excite the sample are not critical. However, if the elements involved in the particular analysis are known, the energy levels may be selected to obtain a maximum number of the characteristic or K X-rays relative to the X-rays of the continuous spectrum.

It should be emphasized that an outstanding advantage of the methods and apparatus of this invention is that the excitation source may be extremely compact and may be mounted on or within the sample to excite fluorescence. Such is not possible with the prior art methods. In addition, of course, the excitation source may be very inexpensive, portable and safe.

An object of this invention, accordingly, is to provide improved methods and apparatus for X-ray fluorescence analysis utilizing radio-active material as a source of energy for exciting X-rays in a sample.

Another object of this invention is to provide improved methods and apparatus for X-ray fluorescence analysis in which the energy levels of X-rays generated by a sample are directly measured.

A further object of this invention is to provide improved methods and apparatus for X-ray fluorescence analysis in which Beta ray-emitting radio-active material objects, features and advantages which will become more fully apparent from the following detailed description taken in conjunction with the accompanying drawing which illustrates preferred embodiments and in which:

Figure 1 is an isometric view of an arrangement for X-ray fluorescence analysis according to the principles of this invention;

Figure 2 is an enlarged sectional view taken substantially along line II-ll of Figure 1, illustrating one preferred form of excitation source in accordance with this invention.

Figure 3 is a sectional view similar to Figure 2 but illustrating another preferred form of excitation source; and

Figure 4 is a diagrammatic view of another preferred arrangement for X-ray fluorescence analysis in accordance with the principles of this invention.

Reference numeral 10 designates an excitation source for generating rays which are impinged on a sample 11 to generate X-rays therein, the X-rays generated within the sample 11 being received by a detector 12 arranged for determining the frequencies or energy levels of X- rays generated within the sample 11, so that its characteristics are determined.

Any suitable means may be employed for supporting the energy source 10, the sample 11 and the detector 12 in proper physical relation. For one example, the detector 12 may include a generally rectangular box-like housing and the source 10 and sample 11 may be supported from such housing by a frame structure including a pair of parallel horizontal members 13 and 14 projecting forwardly from upper portions of the sides of the housing of the detector 12 and a second pair of parallel horizontal frame members 15 and i6 projecting forwardly from lower portions of the sides of such housing with a pair of upright facing

channel members

17 and 18 disposed respectively between the terminal ends of the members 13 and 1S and the terminal ends of the members 14 and 16, a strut 19 being secured between lower end portions of the

upright channel members

17 and 18. The excitation source 10 may be supported between upper end portions of the

upright channel members

17 and 18 and the sample 11 may rest on the members 15 and 16 or, in the alternative. the structure may rest on the sample to be analyzed with the upper surface of the sample against the lower edges of the members 15 and 16.

To prevent impingement of rays directly from the source 10 the detector 12, a shield 20 may be supported therebetween by bracket members 21 and 22 secured respectively to the members 13 and 14. if desired, the bracket members 21 and 22 could be used to support a sample with rays from the source 10 being impinged on one surface of such sample to generate X-rays therein, ancl'with such X-rays being emanated from the oppositely facing surface of such a sample and impinged on the detector 12.

Another pair of

bracket members

23 and 24 may be respectively supported between the members 13 and 15 and the members 14 and 16 in front of the detector 12 and maybe arranged for supporting a critical absorber 25 between the sample 11 and the detector 12.

As will be appreciated from the foregoing preliminary discussion, an important feature of this invention is in the use of radio-active material as a source of energy for generating X-rays in the sample. In one preferred arrangement, the member 26 may be used by itself, but to prevent radiation in all directions and to confine the radiation to a beam, a shield 27 of sufficient thickness to effectively absorb rays from the member 26 may be disposed partway around the member 26, as illustrated in Figure 2. With this arrangement, the provision of the shield 20 may not be necessary.

- Another preferred construction forthe excitation source .r' .10is illustrated in Figure 3. In this arrangemengarnemher 28 of Beta-ray emitting radio-active material ,issurrounded by a member 29'of a target material preferably having a relatively high atomic number such as, for example, lead, tungsten and the like. The Beta rays from .the member 28 impinging on the target member '29 will cause generation of X-rays in the target 29. Such X-rays will include not only the charcteristic Xeraysof the material of the target 29, but also a continuousspeetrum of X-rays. Such X-rays are impinged on the sample 11 and will cause generation of X-rays therein, 'to be detected :by the detector 12. -A-shield 30 similar-to the shield 27 may extend part way around the target 29 to prevent radiation in all directions and. to etfectively con- ;fine radiation into a beam.

It should again be noted that'the generation of X-rays through the combination of a source of Beta ray emitting radio-activematerial and a target is fully disclosed in my prior filed co-pending application entitled X-ray Source,

US. Serial No. 370,175, now Patent No. 2,797,333, filed July 24, 1953, of whichthis applicationis a continuation- .in-part. The optimum spacing of the source relative .to the sample 11, and the most desirable construction of the source 10 may .be readily determined from the teachingsof .that application. With the source 10 constructed in the manner as illustrated in Figure 3, for example, the thickness of the target 29 should preferably be such that the Beta particles from the member 28 will penetrate to the outer surface of the target 29.

The detector 12 may take any desired form and may for example, be a Geiger-Mueller counter or film. Preferably, however, and in accordance with an important feature of the invention, the detector 12 may be a counter of the type referred to in the art as a proportional counter, arranged for the direct measurement of the energy levels of X-rays from the sample 11. The detector 12 may, for example, comprise a scintillation counter which may comprise a target of fluorescent material on which the X-rays may be impinged to generate a certain quantity of light energy which may be detected by a photo-multiplier tube, the output of which is amplified and detected by any desired means.

As another example, the detector 12 may comprise a hollow tube or the like filled with a suitable gas with the X-rays being impinged on such gas to generate electrons and such electrons may cause current fiow between electrodes in the gas and a source of uni-directional voltage applied between such electrodes, the current flow being measured to determine the energy levels of the X-rays impinged on the gas.

The critical absorber 25 may have a characteristic such that the absorption of X-rays gradually decreases as the energy level of the X-rays is increased until a certain energy level is reached at which point the absorption is sharply and greatly increased. With energy levels higher than such certain energy levels, the absorption may gradually decrease. The energy level at which the absorption is greatly increased is dependent, of course, on the type of material forming the absorber 25.

This absorber 25 may be used in a variety of ways for analysis purposes. To determine whether the sample 11 contains a certain element, the absorber 25 may be of a material having a peak absorption at the energy level of the characteristic or K X-rays of such element. Then the response with such absorber may be compared to the response without such absorber to determine the presence of such element.

It may be noted that with many proportional counters, it may be difficult to accurately distinguish between elements having atomic numbers that are very close together. Thus the counter may indicate the presence in the sample of either one element or another element having an atomic number very close to the atomic number of such one element, but it may be impossible to disttit al-sh' theteh t te nf h 't r a be .5 may lthen be brought into use ;to determine which of such ele- ,IQQHE'S is rpresent. Hence the proportional counter may be used to determinethe approximate analysis of a sample and the eritical absorber may then be used to make a determination only with respect to those elements as to which theredsany-doubt. Accordingly, by the combination ,Qf the ;-pr,OpQ .tional counter with the critical abesorber usable-therewith, a very rapid and yet very aca llfa fi ana ys s mayx a v iljfhersample 1 1, may, of course, take any desired form wl1ethcr so id, l q d or ga e T mp :m y, .for -,exa mple, .be of one metal with a coating of another etal thereon, andthe method and apparatus may be used to determine the thickness of the coating orplating o :such another meta and th y may al o e sed to ttiete tp n oles intsu h ccat n ."Ehe in e tion has, f examp be pplied :in the det rminati n .cf .th thi kn ss o a e pl t n o brass ,filldgil'l the determinatiomof a nickel plating on steel.- ,The invention has also been applied to the measurement of the lead content of gasolines and to the detectt ot of :rnc allic contamina tin ph rm t s- Eigure .4 {illustrates schematically another "preferred arrang men or X ay flu e cen e an ysis, according to the principles of this invention. In particular, a source of energy 31, preferably of a form such as illustrated in Figure 2 or Figure 3, may be used to impinge rays on a sample 32 to generate X-rays therein in the same manner as generation of X-rays in the sample 11 from the source 10. These X-rays generated in the sample 32 are impinged on a surface 33 of a crystal 34. The X-rays are diffracted from the crystal 34- and are impinged on a suitable detector 35 by adjusting the physical relation of the sample 32, the crystal 34 and the detector 35, as by adjusting the angle of the crystal 34, and by noting the output of the detector 35, the composition of the sample 32 can be determined. For maximum efiiciency, the surface 33 of the crystal 34 may be curved as illustrated.

This arrangement as illustrated in Figure 4 is generally the same as that used in the prior art, except that -the source 31 comprises radio-active material instead of the X-ray tubes previously employed. By virtue of this feature, the source 31 may be extremely compact and may be positioned in contact with, or very close to the sample 32 so as to achieve eflicient production of X-rays. Thus, the crystal analysis method may be utilized even in spite of the poor geometry attendent thereto, but without requiring the generation of X-rays from an X- ray tube as used in the prior art.

It will, accordingly, be apparent that this invention provides X-ray analysis methods and apparatus which are very efiicient, accurate and versatile, very inexpensive, simple and compact.

It will be understood that modifications and variations may be effected without departing from the spirit and scope of the novel concepts of the present invention.

I claim as my invention:

1. In a method of X-ray fluorescence analysis of a sample, the steps of exciting the sample to generate X- rays therein, and directly measuring the energy levels of X-rays generated by the sample. t

2. In a method of X-ray fluorescence analysis of a sample, the steps of exciting the sample to generate X- ravs therein, and directly measuring the energy levels of X-rays generated by the sample with a proportional counter.

3. In a method of X-ray fluorescence analysis of a sample, the steps of positioning radio-active material in such relation to the sample that energy from the material generates X-rays in the sample, and directly measuring the energy levels of the X-rays generated by the sample to determine its characteristics.

4. In a method of X-ray fluorescence analysis of a sample, the steps of positioning radio-active Beta ray emitting material in such relation to a sample that the Beta rays impinge on the sample and generate X-rays therein, and directly measuring the energy levels of the X-rays generated by the sample to determine its characteristics.

5, In a method of X-ray fluorescence analysis of a sample, the steps of positioning radio-active Beta ray emitting material in such relation to a target that the Beta rays impinge on the target and generate X-rays therein, positioning a sample in the path of X-rays from the target to generate characteristic X-rays in the sample, and directly measuring the energy levels of the X- rays generated by the sample to determine its characteristics.

6. In a method of X-ray fluorescence analysis of a sample, the steps of positioning radio-active material adjacent the sample, confining the energy radiated by the material into a beam impinged on the sample generate X-rays therein, and directly measuring the energy levels of the X-rays generated by the sample to determine its characteristics.

7. In a method of measuring the characteristics of a coating on a base, the steps of supporting Beta ray emitting radio-active material in such relation to the coating and base as to cause generation of X-rays by the coating and base, and directly measuring the energy levels of X-rays emitted from the coating.

8. In a method of X-ray fluorescence analysis of a sample, the steps of exciting the sample to generate X- rays therein, disposing a critical absorber in the path of the generated X-rays, and directly measuring the energy levels of X-rays passed from the sample through the critical absorber. i

9. In a method of X-ray fluorescence analysis of a sample, the steps of exciting the sample to generate X- rays therein, directly measuring the energy levels of X- rays-gerierated by the sample, subsequently disposing a critical absorber in the path of the generated X-rays, and

.then measuring the energy of X-rays passed from the sample through the critical absorber.

References Cited in the file of this patent UNITED STATES PATENTS