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262 PHILlPS TECHNICAL REVIEW VOLUME 18

COUNTERS FOR X;.RAY ANALYSIS

by P. H. DOWbING *), C. F. HENDEE *), T. R. KOHLER *) and W. PARRISH *).

621.387.4:545.824 :548.734

Geiger countersçproportional COl{nlers and scintillation counters haveplayed an all-importantrole as radiation detectors in nuclear physics and allied fields for many years. The Geigercounter was thefirst of these to enter the field of soft X-rays, and in recent years the other typesof counters have been introduced. Faced with a choice between the three types, the worker in thefield must know their relative merits and limitations for differeni appliauions. Such an' assess-ment is given in this article, ,on the basis of the physical properties of the counters and ofpractical measurements under a vari~ty of conditions ~*).

Introduction

About 15years ago.Ceiger counters adapted to thedetection of relatively soft X-rays were introducedinto the general X-ray diffractión field as a substitutefor photographic film. The most striking feature ofthe counter tube in this field is its superiority tofilm in the evaluation ofrelative radiationintensities.This has led to the successful development of themethod of "diffractometry" and related procedures.Three articles on this subject recently' published inthis Review may be quoted 1) 2) 3).

The development of the Geiger counter for X-raydetection has been followed in recent years by theadaptation of the proportional 'counter and thescintillation counter to the same purpose 4). Detectorsof this type and their associated circuits, suitable'for X-ray diffractometry and X-ray spectro-chemical analysis, have heen developed and engi-neered in the Philips Laboratories at Irvington inSUCll a form that they can be readily substituted forthe Geiger counter tube on the "Norelco" X-raygoniometer. Each of the three detectors thus' avail-able has its specific advantages and, of course,..*), Philips Laboratories, Irvington-on-Hudson, N.Y., U.S.A.**)The subject-matter of this article was presented in part at

the 3rd Congress of the International Union of Crystall-ography, Paris, July 1954. Cf. also Acta eryst. 7, 626, 1954.

1) W. Parrish, E. A. Hamncher and K. Lowitzsch, The"Norelèo" X-ray diffractometer, Philips tech. Rev. 16,123-133, 1954/55.

2) W.'iParrish, X-ray intensity measurements with countert#UeS',Philips tech. Rev. 17, 206-221, 1955/56 (No. 7-8).

3) W. Parrish, X-ray spectrochemical analysis, Philips tech.Rev. 17, 269-286, 1955/56 (No. 10).

4) Proportional counter:S. C:Curtan, J. Angus and A. L. Cockroft, Phi!. Mag.40,36-52, 194.9.G. C. Hanna, D. H. W. Kirkwood and B. Pontecorvo,Phys. Rev. 75, 985-986, 1949.W. Bernstein, H. G. Brewer Jr. and W. Rubinson, Nucleon-ics 6, Feb. 1950, pp. 39-45.Seintillation counter:F. H. MarshalI, J. W. Coltman and A.!. Bennett, Rev. sci.Instr. 19, 744-770, 1948.

limitations. In fig. 1 the three detectors and thecomplete interchangeable units are shown. Adescription of the ne,\\, detectors will be given andtheir underlying mechanisms and essential charac-teristics' will be disèussed in' this article.

The Geiger coun~er

For the sake of comparison, the design of a Geigercounter tube for X-ray detection, which has beendescribed on several occasions in this Review,.should be briefly recapitulated.

A cross-section of the standard "Norelco' Geigercounter tube.rtype 62019, is shown infig. 2. The tube.consists of a chrome-iron cathode cylinder 2 cm indiameter, along whose axis a 0.75 mm tungstenanode wire is mounted. The tube is filled with argongas to a pressure of 55 cm Hg, and a small amount(less than 0.4%) of chlorine is added as a quenchingagent. Approximately 1400 V is applied betweencathode and anode. X-rays enter the tube axiallythrough an end window of mica 0.013 mm thick.An X-ray quantum will give rise to a dischargepulse ill the tube if it is absorbed in the gas withinthe active volume, which is a cylinder around theanode wire about 10 cm long (practically the whole .length of the tube) and 1.2 cm in -diameter. Whencopper Ka radiation (cÀ, = 1.54 A, quanta of energy8 keV) is used, in a beam not wider than the activevolume, about 60% of the X-ray quanta impingingon the tube window are detected. This percentage 'is called the quantum counting efficiency of thecounter tube, .The quantum counting efficiency depends on the

absorption of the radiation by the active gas column.The effect of absorption by the tube window and bythe adjoining inactive part of the gas column(which, however, in the 62019 type of tube is

1956/57, No. 9 COUNTERS FOR X-RAY ANALYSIS

89727

Fig. I. Interchangeable X-ray detectors for the "Norelco" equipment. a.) Geiger counter,type 62019. b) Xenon-filled proportional counter, type 62031. c) Scintillation counter,type 52245.The proportional and scintillation counter have cathode follower circuits, which are

mounted directly with the counter on the goniometer arm. The complete detector unitsare shown in the upper part of the photograph.

only about 3 mm long) limits the efficiency. Sinceboth the desirable and the undesirable absorptionare a function of the wavelength of the radiation,the quantum counting efficiency depends stronglyon the wavelength. This is a most important factorfor practical applications, as will be discussed below.

;~~~>C A 89845

Fig. 2. Cross-section of "Norelco " Geiger counter tube, type62019. The tube is filled with argon (55 cm Hg) and a smallamount of chlorine. A voltage of about 1400 V is appliedbetween cathode C and anode A, the permissible voltage range("plateau") is more than 300 V wide. X-rays (X) enter axiallythrough an end window W, the active volume of the tube isindicated by shading. The useful life of the tube is practicallyunlimited; even after having counted 10° quanta, a tube ofthis type showed no deterioration of characteristics.

263

The spectral response of the argon-filled Geigercounter tube, type 62019, as calculated from ab-sorption data 5) and which has been confirmed bymeasurements 6), is shown in fig. 3.

The detailed mechanism of quantum detection ina Geiger counter tube can be described briefly asfollows 7). Absorption of a quantum by a gas atom

5) J. Taylor and W. Parrish, Rev. sci. Instr. 26, 367-373,1955(No.4).

") W. Parrish and T. R. Kohier, Rev. sci. Instr. 27, 795-808,1956 (No. 10).

7) See for instance: D. H. Wilkinson, Ionization chambers andcounters, Cambridge Univ. Press, 1950. Also: N. \XTarmoltz,Philips tech. Rev.13, 282-292, 1951/52. ~ It should be notedthat Geiger counter detectors for nuclear experiments inwhich f3-rays or very hard X-rays (y-ray quanta of millionsof eV energy) are to he measured, have a somewhatdifferent mechanism: the quanta in these cases are absorbed~ generally to a rather low percentage ~ in the cathodewall or in the anode, not in the gas. The development ofthe discharge is similar to that in the detector for softX-rays described here.

264 '.. " PHILIPS TECHNICAL REVIEW VOLUME 18

results in the ejection of a photo-electron. The gasion so produced gets rid of its excess energy in mostcases by emitting C!neor more additional high-energyelectrons (internal conversion by the Augerprocess).·

MoKa CuKa. CrKa.

80

• . • ,

..'" V- ......

~- / <.///

100%

60

20

oo 0.5 1.0 1.5 2.0-I.89673

Fig. 3. Quantum counting efficiency as.it function of wavelength(spectral response) for the argon-filled Geiger counter, calculat-ed from absorption data 5).

These conversion electrons as well as the photo-electron in turn spend their kinetic energy by form-ing ion pairs in the gas. Thus, a total number ofabout 310 ion pairs are formed in argon by theabsorption of one copper Ka quantum. Each of thesecondary electrons will give rise to a Townsendavalanche owing to acceleration in the electricfield" of the counter tube. Metastable atoms and

. photons emitted from the avalanches cause theemission of still more electrons throughout thetube, so that a self-sustained discharge filling thewhole volume of the tube is started. The dischargeis quenched after a short time either by the actionof an external quenching circuit causing the tubevoltage to drop as the discharge current is built up,or by the action of the' quenching agent added to thegas (self-quenching). . .The latter action can be explained as follows.

Most of the ionization, occurs near the central wire,where the electric field strength is largest. Theelectrons thus formed are quickly attracted to(collected at) the central wire, leaving behind apositive ion spaoe-charge sheath around the wire,which effectively lowers the electric .fieldstrength inthe gas and momentarily prevents further ionization.The positive ion sheath drifts out from the wire,restoring after a short time ("dead tiine") theoriginal .field conditions. This "wouldallow the dis-charge to be reinitiated by other electrons subse-quently emitted from the wall; such an emission,however, is prevented by the quenching agent.

2.SA

The gas amplification process described resultsin discharge pulses of large amplitude, say 1-10 V,a feature which makes the Geiger counter an ex-tremely useful radiation detecting device for manyproblems. The "dead time", on the other hand,limits the usefulness of this device. During thistime another quantum ê'ntering. the tube cannot .give rise to a separate pulse and so is not detected.As a result a proportion of the arriving quanta isnot counted, and this proportion increases withtheir arrival rate, i.e. the response becomes non-linear at large radiation intensities. The dead timeof the Geiger counter type 62019 is l70 fLsec;the"effective" dead time for CuKa-radiation producedby anX-raytube operated on.a35kV(peak) full-wave

. rectified voltage 8) is 270 fLsec,so that appreciabledeviations from linearity (> 8%) occur at countingrates as low as 500 counts per' second. Individualintensity values (peak values of diffraction lines)can be corrected for the non-linearity effect. Nopractical correction is possible, however, when thenumber of quanta is measured during a period ofvarying radiation intensity, as, for example, in themeasurement of "integrated line intensities" 9).

The proportional counter

The large pulse size and the long inactive periodof the Geiger counter both originate from thespreading of the discharge throughout the volumeof the tube. In the proportional counter, whosedesign is basically similar to that of the Geigercounter, the voltage on the tube is chosen belowthe threshold value for the self-sustained discharge(i.e. the lowest limit of the Geiger plateau), and thedischarge is then essentially confined to a planenormal to the axis of the tube, located where theoriginal X-ray quantum was .absorbed. The dis-charge current in' this case is only the sum of theTownsend avalanches of the secondary electrons.The "gas amplification factor" therefore is about104, as against 106 or 107 in the Geiger countertube, and the pulses produced are rather small.Because of the limited extent of the discharge,however, nearly the whole volume of the tube re-mains active and can detect a second X-ray quan-tum absorbed shortly after the first. In fact, twoquanta need be separated only by about 0.2 fLsec,i.e. the electron collection' time, in order- to' beseparately detected. When the average current ofthe counter tube is used as a measure of the quantumarrival rate, it is even unnecessary to have separated

8) Loc. cito 2), p. 218.B) Loc. cito2), p. 216.

1956/57, No. 9 COUNTERS FOR X-RAY ANALYSIS 265

pulses; linearity is then maintained up to extremelyhigh counting rates, say 107 counts/sec or more.(At still higher rates, the increasing space chargelowers the effective field.)

Since the pulses produced in the proportionalcounter are approximately 103 times smaller thanthose ofa Geiger counter, an amplifier having a gain.of approximately 103 must be added in order tobring the pulses to the level required for operatingthe counting circuits (about 0.25 V). Evidently,this is the price paid for better linearity.

xFig. 4. Cross section of "Norelco" proportional counter,type 62031.A anode wire, C cylindrical cathode. This counter'is filled with xenon (32 cmHg); a small quantity of methane isadded as a discharge-limiting agent. About 1700V is applied,the. required voltage being dependent on the radiation measur-ed. X-rays X enter through a side window W. Unabsorbedquanta leave the tube through a window diametrically oppositethe entrance window. (Not all proportional counters are fittedwith this exit window.)

A· well-known property of the proportionalcounter, which is responsible for its name, is theproportionality between the size of the pulse andthe' energy of the absorbed quantum. In fact,whereas the pulses of a Geiger counter vary onlyslightly in size, the original quanta acting merelyas triggers, the pulses delivered by a proportionalcounter each represent a charge equal to a fixedmultiple (the gas amplification factor) of theionization produced by the absorbed quantum, andhence are proportional in size to the quantum energy.This proportionality is a very useful characteristicsince it enables the elimination of undesired radia-tion components by the method of pulse heightdiscrimination. We shall deal with. this point atsome length in this article.The "Norelco" proportional counter pictured in

fig. 1, and whose cross-section is shown in fig. 4, isfitted with a 0.013 mm mica window of size 0.6 X 1.6cm in the side wall of the cathode cylinder, with theX-ray beam entering perpendicularly to th~ wire.This design is adopted to ensure the proportionalityjust mentioned: in order to make the gas amplifica-tion factor for all quanta as nearly equal as possible,the spatial distribution of the electric field should beidentical for all quanta, irrespective of the placewhere they are absorbed. (At the ends of thecylindrical. tube considerable distortions of·' the

89674

equipotential surfaces are unavoidable.) Moreover,the "Norelco" tubes are made approximately fourtimes longer than the diameter, in order to enhancethe uniformity of the field distribution in the vicinityofthe window, and any disturbing effect ofthe micawindow rtselfis avoided by covering its interior wallwith either a very thin film of evaporated gold or athin sheet of beryllium. In some tubes' an exitwindow of size 0.8X 1.7 cm is placed diametricallyopposite the entrance window to allow the UIl-

absorbed portion of the X-ray beam to Ieave the.tube without striking the chrome-iron cylindricalcathode wall and possibly causing X-ray fluores-cence.The unabsorbed portion of the beam of course

should be made as small as possible. This was thevery reason why in the early days of diffractometrythe end window design ~as introduced for the argon-filled Geiger counter X-ray detector, ensuring a longactive gas column. The shorter length of theabsorption path in the side-window design is com-pensated by selecting a heavier gas, in most casesxenon. The proportional counter, type 620~2, filledwith pure xenon to a pressure of 32 cmHg, haspractically the same quantum counting efficiencyfor CuKa as the Geiger counter 62019. In fact, the.efficiency of both tubes is nearly identical in thewhole spectral region between about 0.5 and 2.5 Á;cf. fig. 5. Higher gas pressures are not desirablebecause the operating voltages increase rapidly withpressure and would exceed the range of the "Norel-co" power supply.If the internal gas amplification at which the

counter is operated is increased too far, the tubemay go into continuous discharge. Moreover, evenbefore this point is reached, oscillations in the form

MoKa CuKa CrKa100"10 '+ t

f'""

Y ~/ --- ....._

/~;;1 «0- -:~/v /'

80

60

20

oo 7.0 7.5 2.0

_À2.5.40.5

89675

Fig. 5. Spectral .response of the proportional counter filledwith xenon (curve Xc) and with krypton (curve Kr), calculatedfrom absorption data 6). The dotted line is the spectral responseof the Geiger counter from fig. 3.


. of "after-pulses" occur after each normal X-raypulse. The relative amplitudes of the after-pulsesincrease with voltage and may be sufficient tooperate the scaling circui~, thereby falsifying themeasured counting rate. It is. therefore essentialto keep the internal gas gain below some maximum.value. Ithas been our experience that the operatingvoltage therefore should be chosen such as to pro-duce pulses not e:x:ceeding 1 millivolt. (This shouldapply to the most energetic X-ray quanta - desiredor not :.._ which are absorbed in the counter. ,since, the amplitude of the after-pulses increaseswith the quantum energy.) Hence it is necessaryin practice to use a rather, high external (amplifier)gain. ,"

The after-pulsing phenomena and the difficultiesinvolved ~ using high gain amplifiers (especiallywhen pulse height discrimination is used) can beavoided by the addition of methane or some otherorganic gas to the noble gas of the proportionalcounter. The "quenching" or "limiting" agentsprevent secondary processes and limit the dischargeto a single avalanche. Thus, with the "quenched"proportional counter ("Norelco" type 62031) ahigher gas amplification can be used, which allowsmuch greater flexibility in the equipment andtechnique.

Since the limiting agent will gradually decompose,the quenched proportional counters have a limiteduseful life, whereas pure gas' counters have anextremely long life. The usefullife attained by tubeswith not more than 10% methane, however, isadequate for all practical purposes (at least 1 or 2years, in average practice).

Proportional counters filled with krypton havebeen used for the detection of MoK radiation. Thespectral response curve forthe krypton-filled counter,which is also shown in fig. 5~reveals a better quan-tum counting efficiency in the 0.8 A wavelength, region., due to the KrK absorption edge. The other-wise much lower absorption of krypton is in part.compensated by increasing the gas pressureto 50 cmHg.

Using a krypton-filled proportional counter forthe detection of Mo~(a Xvrays brings to prominencea phenomenon which is of considerable importancefOl;measurements requiring pulse height discrimina-tion, viz. the "escape peak". This will be discussedin the section on pulse height discrimination, wherethe 'concept of "energy resolution" will also beintroduced. Since this concept aswell. as the phenom- .enon of the escape peak are of like Importance inthe scintillation counter, the latter should first bedescribed.

The acintillation counterThc scintillation counter is composed of two basic

elements: a flat fluorescent single crystal, and aphotomultiplier tube (fig. 6). ANal-crystalactivated by Tl (1%) is commonly used for softX-rays. An X-ray quantum absorbed by an atom ofthe crystal prodl:lces a photo-electron and' usually

PT

eQS4b

Fig. 6. Schematic representation of the scintillation counter~ype.5224? The seintillating .NaI.TI-crystal (SC) is mountedm.a l~ght:tlgh~holder, fitted With a glass cover transmitting thescintillation hgh~ to the end of the photomultiplier tube (PT)and an X-ray wmdow (beryllium foil) at the other side. .

-one or more Auger electrons, which on their tracksenergize ~ large number of fluorescent centreslocated at the sites of Tl ions in the crystal lattice.Each fluorescent center thus energized may emit avisible photon. The light so obtained is directed ontothe cathode of the photomultiplier tube by reflectingsurfaces near or on the crystal. For roughly 10 ofthese photons one electron is liberated from thecathode. The photomultiplier owing to its secondaryelectron emission gain of 105 or 106 delivers a currentpulse which. after amplification is suitable' foroperation of the counting circuits.The scintillation counter as adapted to soft X -rays

has characteristics similar to the proportionalcounter in two respects: it has an extremely shortdead time (about 0.2 !lsec), ensuring lin~ar countingup to v~ry high intensities; and its pulse amplitudesare proportional to the quantum energy, thus offer-ing the possibility of pulse height discrimination.The outstanding feature of the scintillation counteras compared to both the Geiger and the proportionalcounter is its high quantum counting efficiency(nearly 100%) and its almost uniform spectralresponse in the wavelength region 0.2 - 2.5A. This isshown in fig. 7 for a crystall mm thick. The quantumcounting efficiency of nearly 100% is due to thepractically complete absorption of the soft Xsraysin the Nal-crystal slab 10). Much thinner crystalsof Nal could be used for X-ray analysis owing toits very high X-ray absorption coefficient. The area

10) The efficiency of scintillation counters in their "native"fiel~, viz. nuclear. physics, is rarely higher than say 1%,owingto the very mcomplete absorption of the hard nuclearradintions in crystals of reasonable size.


of the crystal need be only slightly larger than the'cross-section of the X-ray beam.Other characteristics which make the NaI-crystal

particularly suited for the purpose, are the following.Its spectral emission curve matches the sensitivitycurve of photomultiplier tubes rather well; it has ahigh optical transparency; its .fluorescence decayconstant is very small, about 10-7 see, whichaccounts for the short dead time mentioned above;it is easily grown in single crystals of suffieient sizeand uniform transparency. The one importantdisadvantage of NaI.TI is, that it is extremely'hygroscopic. The Crystal therefore is hermeticallysealed in an aluminum holder, which itself is com-pletely opaque to external visible light. For goodtransparency to the incident X-rays, the front ofthe holder exposed to the beam is provided with a0.13 mm thick beryllium window. The other side ofthe crystal mount has a flat glass plate and is"wrung" onto the photomultiplier tube face, with a~uid film of high refractive index (Dow Corningfluid 200) in order to minimize reflection losses.For the same reason the inner surfaces of holder andberyllium X-ray windoware highly polished. Sinceberyllium is difficult to polish, the side of thiswindow next to the crystal is sometimes coveredwith a very thin (- 1 (i.) bright aluminum foil.(It should be noted that light losses do not affect thequantum counting efficiency, but do impair theenergy' resolution, see later.)

MoKa CuKa CrKa

100%'~' ..

(V --I----I

_----_/ ......../ ............////

/

///

/

" '"'"A '"/1 '"-",/ :.,...

80

60

20

o o 7.0 1.5 2.0_Ä

os89676

Fig. 7. Spectral response of the NaI.TI scintillation countertype 52245, calculated from absorption data 6). The dottedline is the response curve for the xenon-filled proportionalcounter from fig. 5.

The decrease of the spectral response curve of thescintillation counter in fig. 7 towards short wave-lengths is due to the harder X-rays passing through'the thin crystal without being absorbed. The de-crease towards longer wavelength, however, is

caused by the increasing absorption of the berylliumwindow. Windows of better transparency at wave-lengths greater than 3 A are conceivable, but for thevery soft X-rays the ~cintillation counter is difficult'to use because the pulses become very small and itbecomes difficult to distinguish them from the noisegenerated by the photomultiplier tube.From this it will be clear that the photomultiplier

tube should be designed for a very low noise ("darkcurrent" caused e.g. by ohmic leakage, thermionicand field emission, gaseous ionization). Coolingthe photomultiplier in order to reduce thermionicemission is not necessarily of benefit in reducing thenoise, since in many photomultiplier tubes the noisepulses of largest amplitude, and hence the mosttroublesome ones, arise from gaseous ionization.

Other characteristics of the photomultiplier, suehas the electron yield per incident photon and thetotal electron gain, have been mentioned above.Suitable types of photomultiplier tubes requireanode voltages of 850 to 1400 V. :The scirrtillationcounter shown in fig. lis operated with an amplifierof gain 2~0 to 1000, depending on the voltage appliedto the photomultiplier tube. The useful 'life of thescintillation counter is practically unlimited.

Pulse height discrimination

Pulse height discrimination as applied in X-raydiffractometry and X-ray spectrochemical ànaly:sishas the object of measuring radiation of a singlewavelength, i.e. one specific diffraction line or onespecific spectral emission line, while radiation of'other wavelengths is eliminated 11). The pulsesdelivered by a proportional or scintillation counterare fed via a linear amplifier (output pulse pro-portional to input pulse) with adjustable gain to asingle-channel pulse height analyzer. ,This may beset to pass all pulses greater than a given amplitude(base) or only those pulses lying within a specified,rather narrow amplitude range (channel width or"window"}, The pulses passed by the analyzer arefed to a recording counting-rate meter 2).

A striking example illustrating the usefulness ofpulse height discrimination is found in the diffractom- ,etry of radioactive samples 12). Owing to the largesize of the specimen 'used in diffractometry, thedetector in the case of a radioactive sample is

11) The general methods of pulse-amplitude analysis and theapparatus used cannot be described in this article. See e.g.:A.. B. 'van Rennes, Pulse-amplitude analysis in nuclearresearch, Nucleimies 10, July 1952, pp. 20-27, and August1952, pp. 22-28. See also 6).

12) D. J. KnowIes, 10th Pittsburgh Conf. X-ray Èlectr. Diffr.1952,p. 32. T. R.KohIer and W. Parrish, X-ray diffractom-etry of radioaètive samples, Rev. sci. Instr. 26, 374-379,1955 (No. 4). '


exposed to an intense background radiation.Shielding of the detector by lead (and scanning bymoving the X-ray tube, with its heavy shield,instead of the detector) is a possible but cumber-some solution. If, however, a scintillation or pro-portional counter is used, the vast majority ofpulses produced by the (3- or y-rays have amplitudesdifferent from those produced by the diffractedX-rays (they may -be larger as well as smaller);they can therefore be largely eliminated by a pulseheight discriminator with the window set for theenergy (wavelèngth) of the diffraction line whoseintensity is to be measured.

Energy resolution of different detectors

, When a given radiation is being measured whilethe upper and the lower amplitude limits of theanalyzer windoware continuously shifted insynchronism with the recording chart (keeping aconstant window width), a graph of the. pulseamplitude distrib,ution of the pulses of the detectoris obtained. Fig. 8 shows the pulse amplitude

(\

)I\J \15V 0 5 10 15V 0

12.la

s:1555 la

Êo

89725

Fig. 8. Amplitude distribution curves of the pulses generatedby CuKa radiation in a) krypton-filled proportional counter,b) xenon-filledproportional counter, c) scintillation counter.

distribution curves, obtained with a window 0.5 Vwide, for three types of detectors exposed tomonochromatic radiation (CuKa, which gives 10 Vpulses). Each curve exhibits a peak of finite width,which means that even quanta of a single energy(i.e. 8 kcV) produce pulses of different amplitudes,varying around an average value; this valuecorresponds to the true quantum energy. The varia-tion, which limits the power of the detector toseparate quanta of closely spaced wavelengths, isdue to the statistica of the ionizati~n or excitationand amplification processes involved. The separat-ing power, or "energy resolution" (which of coursecannot be enhanced by a narrower analyzer window)is measured by W/A, W being the width at one half

20V

peak height of the pulse amplitude distributioncurve and A the average pulse amplitude (roughlythe abscissa of the centre of the peak).

In fig. 8 it is seen that .the energy r'esolutionW/A for CuKa radiation is much better for theproportional counter, filled with xenon or krypton,than for the scintillation counter. The betterresolution of the propor-tional counter can beunderstood as follows. One CuKa quantum willproduce an average number m of 350 ion pairs in thexenon-filled proportional counter. Owing to fluctu-ations of m and of the gas amplification factor foreach electron, the resulting pulse amplitude fluctu-'ates with a relative standard deviation a/A, whichcan be shown 13) to amount-to about ll/m. Usingthe relation W = 2.36 a, the ratio W/A is calcu-lated to be 13% for' m .= 350. The actual valuemeasured in mass-produced counter tubes is about20% (irregularities of the anode wire and slightasymmetry in its centering, which cause fluctuationsof the gas amplification, probably account for thedifference). In the scintillation counter, one CuKaquantum will produce about 500 visible photonsin the NaI.TI crystal ~4), but only about n = 25of these photons give rise to an "electron avalanche"in the photomultiplier tube. (The other photonsare ineffectual because either they do not arriveat the photocathode, or they fail to liberate an elec-tron, or the liberated electron fails to reach the firstdynode, or because of fluctuations in the secondaryemission process at the dynodes 15).) The relativestandard deviation again is a/A R:j INn = lf1!25, sothat W/A = 2.36 a/A = 47%. Thus the ratio W/Afor the scintillation counter is in this example about2.5 times larger and its energy resolution about 2.5times smaller than that of the proportional counter.Significant improvements in the energy resolutionof both these types of detectors do not appearlikely at present.

From die relations Woe lfl1m a~d Woo lfl1nimplied in the above it is seen that for both types ofcounter tubes W/A oe lfl1A: the energy resolutionimproves with the square root of the quantum energy .

13) U. Funo, Phys. Rev. 72, 26-29, 1947; O. R. Frisch (un-published); S. C. Curran, A. L. Cockroft and J. Angus,Phil. Mag. 40, 929-937, 194·9.

B) About 20% of the quantum energy is converted into visiblelight; the quantum wavelength is 1.54 Á, the averagewavelength of the visible emission is 4-100 Á, so that thenumber of visible photons per X-ray quantum is 0.2 X4100/1.54 ~ 500. - It should perhaps be noted that thelimited energy-conversionefficiency (20%) does not detractfrom the nearly 100% quantum counting efficiency of thescintillation counter for CuKa, mentioned earlier: practical-ly every absorbed quantum will be counted anyhow,independent of the loss of large part of its energy in thecrystal. ,

15) G. A. Morton, RCARev. 10, 525-555, 1949.


of the detected radiation (i.e. - within obviouslimits - inversely with the square root of its wave-length).

When the composition of the radiation to bemeasured' and the behaviour of the' detector are notknown, the desirable setting of the pulse heightanalyzer window may be quickly determined byusing only the base of the analyzer (i.e., with theupper limit of the window inoperative, see above)and making a reeording with synchronized baseshift and chart movement. This will yield what iscalled the "integral curve" [number of pulses persecond having an amplitude exceeding the valueon the axis of the abscissa). .The integral curvesobtained for CuKa radiation with the same three, detectors as in fig. 8 are reproduced in fig. 9.

o 5 10 15V 0 5 10 15V 0

ê I!.

5 10

s:Fig. 9. Integral curves of the same pulses as in fig. 8. Theordinate of each curve indicates the number ofpulses per second ,having an amplitude exceeding the voltage value plotted asabscissa.

The escape p-eak

The primary process in the absorption of an X-rayquantum by all the counter tubes considered is theejection of an electron from an inner shèll of a gasor crystal atom. This photo-electron will' originatesay from the K shell, if the wavelength of the ab-sorbed quantum is shorter than that of the Kabsorption edge of the atom. The following transi-tion of an L shell electron to the K shell vacancy isassociated with either the ejection of one or moreother electrons (Auger process) or the emission of anew X-ray quantum (fluorescence). It has beenstated above that the Auger electrons similarly tothe photo-electron will ultimately he absorbed in thegas, or crystal, and will contributs to the formation'of ion pairs, or to the excitation of fluorescentcentres. The alternative fluorescent X-ray quantum,however, has a sizable probability of escaping fromthe gas or crystal, especially since its wavelength

20V

is always somewhat longer than that of the absorp-tion edge and absorption for these quanta is there-fore relatively weak (cf. figs:5 and 7'). For the Geigercounter it does not matter greatly whether a 'sub-stantial proportion of the original energy Ei of thequantum manages to escape as fluorescent energy,Eç, For the proportional and scirrtillation counter,however, the escaping of fluorescent X-radiationhas the undesirable consequence that some of theoriginal quanta 'result in pulses with a "wrong"amplitude, viz. proportional to Ei - Ef instead ofEi. Thus two peaks appear in the pulse distributioncurve for monochromatic radiation, the main peak(at Vi ocEi) and the "escape peak" (at VeOCEi- Ef).A small escape peak is visible in fig. 8b for CuKa

radiation incident on the xenon-filled proportionalcounter. A very large escape peak - larger thanthe main peak - is found when MoKa radiation(Ei = 17.4 keY) is measured with a proportionalcounter filled with krypton (the escaping fluores-cent KrKa radiation has Ef = 12.6 keY). This isillustrated by.fig. la.

The existence of the eseape peak will influencepractical measurements with pulse height discrimi-nation in two ways. Since theanalyzer window usu-ally has to he made so narrow that it will rejectpulses of the escape peak, the quantum countingefficiency will be correspondingly reduced: The losswould amount to about 55% in the case of Jig. la.Secondly, for quanta Ei + Ef, if present, the escapepeak would inevitably be passed by the analyzerwindow set for the main peak of quanta Ei; thediscrimination against unwanted radiation (e.g.a continuous background) is therefore incomplete.

89726

H96f}

Fig. 10. Amplitude distribution of the pulses produced in thekrypton-filled proportional counter by l\:IoKaradiation (quan-tum energy Ei = 17.it keV). The pulses forming the "mainpeak" have amplitudes (average 32 V) corrcsponding to theionization produced by both photo-electron and Auger elec-tron(s). The amplitudes of the other group of pulses (average9 V), forming the "escape peak", correspond to the residualionization when fluorescent Xsruys are lost. The amplitudedifference (23 V) between pulses of main peak and escape peakis determined by the energy Er of the escaped fluorescentX-ray quantum (KrKa, with Er = 12.6 keV) and thereforeremains constant when the wavelength of the detected radia-tion is varied.

270 PHILIPS TECHNICAL REVIEW VOLUME 18

Jn some cases the escape peak can be put to good use,since the energy resolution of the counter is better in this partof the pulse amplitnde distribution curve than in the mainpeak. This can be seen as follows. Consider the problem ofseparating, in spectrochemical analysis, two closely spacedemission lines, of quantum energies Ei, and Ei2' and assumethat the detector delivers an esca pe peak for each line, the ener-gy of the escaping fluorescent quantum being Er. The meanpulse amplitude Ac of the escape peak in each case is smallerthan that of the main peak Am by the same amount L1A,proportional to Er. Thus the escape peaks of both radiationshave the same separation as the main peaks, ACl - AC2 =Am, - Am2. The half height width W of any peak in the pulseamplitude distribution, however, is proportional to the squareroot of the mean amplitude, ]IA (see above). Each escapepeak, therefore, is narrower than the main peak by a factorIIAc/Am. Evidently the narrower escape peaks with equalseparation can be more easily resolved than the main peaks.

quantum counting efficiency of the proportional counter is low

at these short wavelengths.Incidentally, it is seen in this recording that one radiation

may produce several escape peaks. In fact, besides Ka fluores-cent quanta, K{J quanta and in other cases L quanta may arise,with their corresponding fine structure. Usually these niceties

can be disregarded.

Comparison of the three types of detectors inpracticalapplications

When using the "N orelco" X-ray instrumentation,it is necessary to choose for each problem the mostsuitable detector among the three available types.The considerations determining this choice will nowbe discussed. In doing so, it seems logical to startwith X-ray spectrochemical analysis: shortcomings

Xe Pr NdAbs; Ka. Ka.34.6 36.1 37.4-

Fig. 11. Illustration of the fact that the resolving power of a counter is better in theescape peak region than in the main peak region. The pulse amplitude distribution curvesA and B were obtained from a xenon-filled proportional counter with monochromaticradiations of energies 36.1 keY (praseodymium Ka) and 37.4 keV (neodymium Ka)respectively.

Use can be made of this fact by setting the window of the pulseheight analyzer for one of the two escape peaks rather than for

one of the two main peaks.A striking example is seen injig. 11, which shows the pulse

amplitude distribution curves ofaxenon-filled proportionalcounter for the Ka radiations of praseodymium (atomicnumber 59, Ei = 36.1 keY) and neodymium (atomic number60, Ei = 37.4 keV). Both radiations have wavelengths shorterthan the K-absorption edge of xenon and produce a verystrong escape peak (85% of the total number of output pulsesare grouped in this peak l). Since Er = 28.9 keV for Xe Ka, it iscalculated that the escape peak width is about 0.5 times themain peak width. It should be noted, however, that the

89728

of the Geiger counter for this particular applicationgave the fillip to the introduetion of proportionaland scintillation counters into the X-ray field, sothat here the case is rather clear-cut.

Spectrochemical analysis 3)In spectrochemical analysis, radiations of dif-

ferent wavelengths, whose intensities may varygreatly, must be compared quantitatively or semi-quantitatively. The non-linearity of the Geigercounter due to its long dead time is a serious dis-


advantage in this case, since it will generally necessi-tate a correction of the measured peak intensities.Such a correction is neither easy nor accurate,particularly because the operational dead time ofthe counter when the X-ray tube is not suppliedwith a constant potential but e.g. with a full-waverectified voltage, will depend on the wavelength 16).For intensities exceeding a few thousand counts/sec(intensities exceeding 105 counts/sec may occur),corrections are no longer possible and it willbe necessary to reduce the intensities to thcGeiger counter range by controlled absorption.' Thisbecomes very difficult for a series of different wav~-lengths. ,

Both the proportional and the scintillationcounter have much hetter linearity than the Geigercounter. Their dead time (~0.2 fLsec)would causenon-linearity only at 107 counts/sec or even higher.The practical limits of linearity in this caseare set by the associated electronic circuits. Thestandard "Norelco" scaling circuit gives rise to acount loss of 10% at 14000 counts/sec. New circuitsnow available are linear to 1% 'at 10000 counts/sec.Similarly the pulse height analyzer hitherto used inour investigations precluded the measurement ofcounting rates higher than' about 5000 counts/sec,owing to a shift of the base level depending on thecounting rate. Better analyzers are now availablewhich retain the simplicity ofthe present instrumentbut permit intensity measurements up to about 15000'counts/sec or even higher,' depending upon the count-ing rate of the pulses being rejected by the analyzer.

The strongly wavelength-dependent quantumcounting efficiency, which the proportional countershares with the Geiger counter (fig. 7), requirescorrections to the measured intensities when charac-teristic lines of elements several atomic numbersapart are compared and no reference standards areused, as is frequently done in qualitative and semi-quantitative analysis. Although there are several. other factors which influence the relative line intensi-ties, it is an important advantage of the scintillationcounter for spectrochemical analysis that owing to itsnearly uniform spectral response, corrections torvariations in the efficiency are not necessary. Evenmore important is its very high value of the quanturncounting, efficiency, particularly for wavelengths< 1 Á. This advantage is directly reflected in thetim~ necessary for each measurement, since a fixed

16) For a given peak voltage on the X-ray tube, the fractionof the a.c. cycle during which the elements in the specimencan fluoresce and into which, therefore, the quanta to becounted are bunched, decreases as the atomic number

, increases. Thus the effective dead time becomes longerfor regions of shorter wavelengths.

number of counts must be accumulated for a givenaccuracy (cf. 2)); the counting rate obtained withthe scintillation counter exceedsthat of the xenon-filled proportional, counter by a factor of almost 2for CrKa and of about 12 for AgKa.

For long wavelengths the scintillation counterloses its advantage owing to the rather large photo-multiplier, noise mentioned previously. Measure-ments with all types of counters are hampered hereby the difficulty .ofmaking sufficiently transparentX-ray windows, Using non-vacuum-tight windowsin special flow counters' has extended' the usefulrange of the proportional counter with its inherentlow noise to about 12 Á (in a helium path goniom-eter, cf. 3)): Ka lines of silicon (7.13 Á), aluminum(8.34 Á), magnesium (9.89 Á) and sodium (11.91 Á)could thus be measured with a' good peak-to-back-ground ratio 17), whereas the longest wavelength forwhich we were. able to make useful measurementswith the scintillation counter, using a speciallyselected crystal and photomultiplier tube, was Kaof sulphur (5.37 Á).'TiKa (2.8 Á) is about the longestwavelength that can be measured in routine fashionwith the standard "Norelco" scintillation counters.. The possibility of pulse height discriminationwith the scintillation counter and proportionalcounter is a great asset in spectrochemical analysiswhen peak intensities become smaller.' Thus theanalysis of minor and trace amounts of elements isachieved with better accuracy and lower limits ofdetectability. The usefulness of pulse height dis-crimination for separating overlapping spectrallinesof different elements has been discussed in a previous~rticle 3), as well as the possibility of a non-dispersivespectrochemical analysis based on those principles.Clearly the greater energy resolution of the propor-tional counter may be advantageous in these cases,although one must balance this advantage againstthe higher quantum counting efficiency of thescintillation counter for short wavelengths.

Powder diffractometry 1)

The situation in diffractometry is quite differentfrom that in spectrochemical analysis, for two rea-sons: the intensities to be measured 'are usuallymuch lower, and in principle use is made of mono-chromatic radiation, e.g. CuKa (1,54 Á) or MoKa(0.71 Á) or CrKa (2.29 Á). The Brägg reflections of-this radiation obtained from the specimen (sharp

17) C. F. Hendee, S. Fine and W. Brown, Rev. ,sci. Instr. 27,531-535, 1956 (No. 7). Prof. C. H. Shaw of Ohio StateUniversity, according to a personal communication, hasreached about 'the same limit by using proportionalcounters with a beryllium foil window.

272 PHILIPS TECHNICAL REVIEW VOLUME 18

Table J. Comparison of detectors for powder diffractometry. l\feasure.:nents performed on the (111) reflection of a pure siliconpowder specimen (no fluorescent background). For each of the three X-ray tubes (target 1\:[0, Cu and Cr respectively) the voltagewas empirically selected so as to givc the highest ratio of useful characteristic to undesired "white" radiation. The pulse heightanalyzer window was set for a transmission factor of about 0.90. The Geiger counter measurements were corrected for dead timelosses (1' = 270 (lsec). The figures of merit are relative values, with the first in each group arbitrarily taken as 100.

lines) are superimposed on a hackground consistingchiefly of two contributions: radiation of a' continu-ous spectrum, which is unavoidably emitted by theX -ray tube and scattered by the specimene andcharacteristic fluorescent X-radiation 'excited in thespecimen 18). The scattered continuous backgroundhas its maximum intensity at wavelengths usuallymuch shorter than the useful radiation. A fluor es-'cent background - which will appear only if thespecimen contains elements whose excitation voltageis sufficiently low - has wavelengths shorter orlonger than the useful radiation. The total intensityand the spectral distribution of the background thusmay vary widely, depending on the specimen, theX-ray tube voltage and other experimental condi-tions.

Counter tubes for powder diffractometry shouldhave a high quantum counting efficiency, because ofthe low intensity of most diffraction lines, and shouldyield a high peak-to-background' ratio, for the'measurement of weak lines. Linearity it usually ofless importance, and so is the uniformity of thespectral response. The non-uniform spectral responseof the Geiger counter (and proportional counter)in fact may be regarded as an advantage for someapplications of powder diffractometry, since itproduces a' crude sort of monochromating effect,which eliminates a large part of the short-wavelength

18) It is assumed that non-specimen scatter, due to misalign-ment of the goniometer, is practically eliminated. Thecontribution of'.uir scatter can usually be neglected, Inaddition to the continuous radiation, a portion of the usefulmonochromatic radiation is scattered (not "reflected") bythe specimen. This contribution to the background ofcourse cannot be distinguished from the dilfraction lineproper by pulse height discrimination. - A possiblenon-X-ray background, as in the dilfractometry of radio-active samples 12), need not be considered here.

continuous scattered background. A qualitativeconclusion is that for many applications of diffract-ometry, especially when simplicity of equipmentmatters, the Geiger counter offers a verY'satisfactorysolution, Nevertheless, the scintillation counter;using more elaborate equipment, is capable of evenbetter peak-to-background ratios: the lack of theabove-mentioned inonochromating effect is over-come by pulse height discrimination. The highquantum counting efficiency of the scintillationcounter is another advantage. This efficiency, onthe other hand, is diminished when pulse heightdiscriminatien is applied: the pulse height analyzerwindow, set for the peak in the pulse amplitudedistribution curve for the monochromatic radiationused (fig. 8), will transmit only a certain percentageof the relevant pulses owing to the finite peak width.The quantum counting efficiency multiplied by the"transmission factor", will be called the "detectionefficiency". Making the window wider Will increasethe transmission factor and therefore the detectionefficiency, but reduce the monochromating effect.

From the above it will be clear that for choosingthe best detector for any particular application ofpowder diffractometry, a quantitative comparisonof the detection efficiency and the peak-to-back-ground ratio obtained with each of the three typesof detectors is necessary. Giving equal importanceto these two quantities, their product may becalculated and used as a ''figure of merit" of thedetector. A large number of measurements havebeen made in order to establish these figures ofmerit for a variety of cases 6).

As a first instance, a typical inorganic specimenwas selected and measurements were made usingthe three radiations previously mentioned abovc.

MoKa,SS kV!" 7 mA; CuKa, 40 kVp, 9.6 mA; CrKa, 30 kVp, 12.8 mA;filter Zr 0.002": filter Ni 0.0007"; filter V205;

peak P at 20 = Ü.Oo; peak P at 28 = 28,4.°; peak P at 20 - 42.9°;background B at 10.0°; background B at 20.0°; background B at 35.0°;

Detector bea~ apert: 2a = 0.5° beam apert. 2a = 1.0° I beam apert. 2a = 2.0°, Figure Figure Figure

Det. elf. 'PIB Det. elf. Det. elf.%

of % PIB of % PIB ofmerit merit merit

Scintillation counter with pulse heightdiscrimination 90 44 100 89 134, 100 90 93 100

Seint, counter without pulse ht. discr. 100 10 25 100 12 10 100 5 6

Prop.c. (Xe) with pulse ht. discr. 12 44 13 56 146 69 58 92 64Prop.c. (Xe) without pulse ht. discr, 13 16 5 63 57 30 60 16 I!Prop.c. (Kr) with pulse ht. discr. ,12 51 15 33 64 18 - - -Prop.c. (Kr) without pulse ht. discr. 29 29 21 36 26 8 - - -Geiger counter 14, 27 • 10 I 51 46 20

I59 18 13

1956/57, No. 9 COUNTERS FOR X·RAY ANALYSIS 273

Fig. 12. Spectrum from Cu- target X.ray tubc (40 kVp, full-wave rectification) obtainedwith scintillation coun tcr and silicon « Ill) plane) analyzer crystal, a) wi th out, b) with pulsehcight discrimination, the window being set Ior CuKa with a transmission factor of 0.90.A Ni-filter 0.0007" thick, which virtually eliminates the CuK;'i line and also reduces thecontinuum, was employed in both cases. The nearly complete suppression of the continuousspectrum in (b) is responsible for the enormous improvement in the peak-t.o-backgroundratio in diffractometer measurements of polycrystalline specimens with CuKa, as shown inTable I (increase from 12 to 134). Incidentally, the small residual hump at about 0.35 Á isdue to the escape peak phenomenon: radiation of wavelength ~ 0.35 Á owing to thisphenomenon will produce a certain proportion of pulses falling within the analyzer windowset for 1.54 Á. .

The specimen gives no measurable fluorescence withany of these radiations. The results are listed inTable J. For MoKa the superiority of the scintilla-tion counter over the other detectors is very marked;this evidently is due to its much higher quantumcounting efficiency at this wavelength. The figureof merit of the krypton-filled proportional counteris seen to he lowered when pulse height discrimina-tion is applied. This fact is caused by the strongcscape peak, whose supprcssion reduces the dctec-tion efficiency more than is gained in the peak-to-background ratio. For CuKa and CrKa the scintilla-tion counter with pulse height discrimination also

64O~I WIm'!i1t [HHrltIJ I' 1:1 '.'~

I" 1fi:i8 ..120 nillI ;ii, JfI !I" ,. I': :t r:T illillii

r- l"f i911fll1~11IH;i' , . ~.

40 .i RiITffit I", .~ Iw: ~If',~ r1I~-

r:r " IW. :.lilll III.]-., î= 1':; I:' r I""-~ ili Pi; I~ .,560 I

\ ). .!±!J ,,1,.~

.1 , i IHiJWii; , lif fi

280 iT \ Ij.

~

I:.~1\ I; I1

<IHI: IIW i...... l~~ l- f-"0 l~l';1 rm '-r:: 1'1 r;;, r-0.3OS 7.0 7.5 2.0 2.

5

38

2

1

rates as the best detector but not by so wide a mar-gin as in the casc of MoKa. When pulse hcight dis-crimination is not applied, the advantage of theGeiger counter (and of the Xe proportional counter)in not being sensitive to the short-wavelengthscattered radiation is clearly seen; the Kr pro-portional counter has a very low figure of merit inthis case owing to the rise of its quantum countingefficiency at the KrK absorption edge (fig. 5), andfor similar reasons the figure of merit of the scintilla-tion counter without pulse height discrimination is

low for CuKa and CrKa. The effectiveness of pulseheight discrimination in the latter case is very clearlyillustratcd by fig. 12.

It is interesting to note that the peak-to-background ratiosobtained with the scintillation and Xe proportional counterare several times higher for CuKa and CrKa than for MoKa.The reason for this is that the characteristic MoKa line isnearly at the peak of the continnous spectrum, whereas theCuKa and CrKa lines are well removed from the peak 19).

A second series of measurements, which will notbe reported here in detail, was concerned with thescattered backgrounds in the diffractometer patterns

of different (non-fluorescent) specimens, measuredat several diffraction angles. The total backgroundwithout pulse height discrimination varied by afactor of 20 among the specimens tested. The reduc-tion of total background by the use of pulse heightdiscrimination varied from a factor of about 2(specimen hexamethylene tetramine) to 53 (speci-men silver), for the scintillation counter 6). Dis-

19) W. Parrish and T. R. Kohier, A comparison of Xvraywavelengths for powder diffractometry, J. app!. Phys. 27,1215-1218, 1956 (No. 10).


700"10

CuKat, _---_

" \.... ....

/ "/ / " ,/ I \ ,/ ",

/

/ IJ \~ ""/1 ", K""'~/l.......

80

60

a

20

o o89678

0.5 1.0 1.5 2.0-Ä

2.5

100"10. " , ' ---- ----

I' -I I' Ir-,

----- .....

I ....- .... _I

/ CuKa '\III \.II / '\IIII, / \..) <,

/\

80

60

b

20

o o99679

0.5 1.0 1.5 2.0_À

2.5

Fig. 13. Detection efficiency for monochromatic radiations asa function of Î., with symmetrical pulse height analyzer win-dow (transmission factor 0.90) set for CuKa. a) Proportionalcounter filled with xenon. b) Scintillation counter. The dataare calculated from the quantum counting efficiencies(dottedcurves, as in fig.7) and the pulse amplitude distribution curves.

crimination with the Xe proportional counter givesa smaller reduction factor, as may be expected fromits lower quantum counting efficiency in the shortwavelength region.

Finally a number of measuremerrts were madeconcerning the background due to specimen fluor-escence. The diffraction pattern of iron powder ob-tained with Cu Ka radiation was selected as arather difficult case. The background from the

lower level, resulting in an asymmetrical window willdecrease the detection efficiency for FeKa by alarger factor than that for CuKa. A ratio of 25 isobtained' with the asymmetrical window indicatediron specimen contains strong FeKa and FeK{Jfluorescence, in addition to a certain amount ofscattered short wavelength èontinuum. The FeKawavelength (atomic number 26, A = 1.93 A, energy'6.4 keY) is rather near that of CuKa (atomic number29, A = 1.54 A, energyBiû keY). The efficiency ofpulse height discrimination in suppressing the un-desired FeK radiation when the Window is set for

3.0.1t CuKa will in this case largely depend on the energyresolution of the counter. Quantitative evaluationof the effect of discrimination is 'simpler for such acase'than for the suppression of a continuous back-ground, since the quantum counting efficiencies foronly two wavelengths need be considered. The ratioof the detection efficiencies for these two wave-lengths (i.e. CuKaJFeKa) takes the place of thepreviously introduced peak-to-background ratio,and the figure of merit is redefined for this caseas the product of this ratio times the detectionefficiency for CuKa.

The detection efficiency of a counter as a functionof wavelength for a given setting of the analyzerwindow is easily calculated from its spectral responsecurve (fig. 7) and the pulse amplitude distributioncurve for monochromatic radiation (for examplefig. S). The result is shown infig. 13a for the xenon-filled proportional counter, in fig. 13b for the seintil-lation counter. In both cases a symmetrical windowsetting with a transmission factor of 0.90 was used.

The result looks rather disappointing: a CuKaJFeKa ratio of not more than 1.2 is obtained with thescintillation counter and of 1.6 with the proportionalcounter. It is not essential, however, to use the above-mentioned symmetrical setting of the pulse heightanalyzer window. Consider the integral curves ofthe proportional counter for CuKa and for FeKaradiation, ,fig. 14. The symmetrical window settingis indicated by verticallines; a higher setting of the

3.0.8.

Table n. Reduction of fluorescent background with pulse height discrimination (ironspecimen, irradiated by radiation of copper target X-ray tube operated at 40 kV(peak);other experimental conditions see fig. 14).. '

Symmetrical analyzer window Asymmetrical analyzer window

Ratio of Figure Ratio of Figuredetection detectionefficiencies of efficiencies of

CuK<;t/FeKmerit CuKa/FeK

merit

Xe proportional 1.6 13.5 '25 100counter

Scintillation

I I -counter

1.2 16.3 4.1 27.5

• T


Fig. 14. Integral curves of the pulses obtained froin th~ xenon-filled proportional counterwith CuKa radiation (diffracted (lll) peak of a silicon powder specimen) and with back-ground of a pure iron specimen at 20 = 23°; FeK fluorescence is predominant in thiscase. The vertical lines indicate the conventional symmetrical setting of the pulse .heightanalyzer window, (transmission factor 0.90). Experimental conditions: copper target X-raytube, 0.0008" nickel filter for suppression of CuKf3,beam aperture 2a = 1°, receiving slitwidth 0.024". The tube was operated at 20 kV(peak) for silicon and at 40 kV(peak) foriron. The curves were normalized to have the same counting ràte at 5Vpulse amplitude (thisis permissible since only the relative attenuations caused by the analyzer are considered)Fig. IS. Same as fig. 14, but for the scintillation counter. The smaller slope of the curves'as compared with fig. 14 is due to the poorer energy resolution of the scintillation counter;the higher intensity level at pulse amplitudes above about 18V (scattered short wavelengthcontinuum) is due to the higher quantum counting efficiency of this counter at short wave-lengths.

II·1.5 la 20

Fig. 14

lower level, resulting in an asymmetrical windowwill decrease the detection efficiency for FeKa bya larger factor than that for CuKa. A ratio of 25 isobtained with the asymmetrical window indicatedby shading in fig. 14. A similar but less spectacularimprovement is obtained when an asymmetricalwindow is u~ed for the scintillation counter, asshown in fig. IS. The results are summarized inTable 1I.The smaller improvement with the scintillation

counter is a consequence of its poorer energyresolution. On the other hand, even the betterresolution of the proportional counter may proveinadequate when smaller wavelength differences

Summary, A Geiger counter, a xenon- or krypton-filled propor-tional counter and' a scintillation counter are available. asinterchangeable detectors for the "Norelco" X-ray goniometer.The design and counting mechanism of each of these detectorsis briefly described in this article. The main characteristics ofthe three detection methods are: the Geiger counter has largepulse amplitude and concurrent simplicity of equipment; theproportional and scintillation counters have linear responseup to very high counting rates, and proportionality betweenpulse amplitude and quantum energy, offering the possibility

. of pulse height discrimination; moreover the scintillationco:unterhas the highest quantum counting efficiencyand a veryuniform spectral response, whereas the proportional countershave a better energy resolution. After a discussion of someaspects of pulse height discrimination, especially energyresolution and the escape peak phenomenon, an attempt ismade to evaluate the relative merits of the different types ofdetectors for several applications. For spectrochemical analysis,

589680

Fig. 15

occur, as with CoK fluorescence (atomic number 27)excited by CuKa radiation. In that case the use of acrystal monochromator placed after the specimenwould be necessary in order to remove all the fluores-cent background; the pulse height analyzer may thenbe omitted and because of its simplicity a Geigercounter will preferably be used for the detectionof the monochromatic radiation, especially since theintensities in this case will be rather low. This ex-ample is perhaps a fitting conclusion for this article,since it reintroduces the eldest member of thefamily of X-ray counters and clearly illustrates thateach member has specific applications in which it issuperior to the others.

the scintillation counter will in most cases he preferredfor wavelengths up to about 3 Á. For longer wavelengths a verythin window proportional counter (either sealed off or flowtype) is desirable. For powder. diffractometry, the equalimportance of high sensitivity and good peak-to-backgroundratio leads to the proposal of a figure of merit, which is calcul-ated for different counters and for a typical non-fluorescingspecimen analyzed with three radiations (CuKa, MoKa and.CrKa). The scintillation counter used in conjunction withpulse height discrimination has by far the highest figure ofmerit for MoKa radiation; its superiority above the xenon-filled proportional counter is less pronounced for CuKa andCrKa radiation. Finally, a fluorescent specimen (iron powder .

, analyzed with CuKa) is considered. Pulse height discriminationin this case is especially effective when an asymmetricalsetting of the analyzer window is used, and the xenon-filled,proportional counter is markedly superior to the scintillationcounter owing to its better, energy resolution.

counters for x;.ray analysis - … bound... · counters for x;.ray analysis ... w. parrish, x-ray...

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