IEEE Transactions on Nuclear Science, Vol. 33, No. 1, February 1986

QUANTITATIVE ANALYSIS OF ALPHA ACTIVITIES IN THICK SOURCESUSING SI DETECTORS

Y. Takami, T .Hashimoto, F. ShiraishiInstitute for Atomic Energy, Rikkyo University,

Nagasaka, Yokosuka, 240-01, Japan,

and

Keith VossDepartment of Nuclear Engineering, Iowa State University,

Ames, Iowa, 50010, U.S.A.

Abstract

An alpha spectroscopy method for thick sourceswas developed. Alpha energy spectrum measured at aplane surface of a thick source is inverselyproportional to the dE/dx of the medium. Thisprinciple was applied to analyze alpha nuclides in lowlevel environmental samples quantitatively, using alarge area Si Surface Barrier Detector(SBD).

Introduction

In reactor facilities or alpha emitter handlinglaboratories, alpha activities in various forms mustbe monitored for the surface and air contamination inthe working area, radio-active wastes, andenvironmental samples. Gross alpha activitymeasurement, gamma spectrometry and alpha spectrometryafter chemical separation are commonly used as themonitoring methods.

Alpha spectrometry employing chemical separationrequires complicated technique, high cost,longprocessing time, and correction of the chemical yield.

Alpha detection by gamma spectrometry isineffective for alpha nuclides of low gamma emissionyields. The development of simple and rapid alphamonitoring methods is greatly needed in the alphaemitter handling laboratories.

Utilization of Si detectors for reactorfacilities has been developed in our Institute. Andlarge area p-type Si detectors werj fabricated foralpha- and beta-ray monitoringi . This paperdescribes a new method of non-destructive analysis forlow level alpha activity samples, utilizing the largearea Si detectors. The pulse height distribution ofalpha particles emerging out of an infinitely thicksample surface of fine powder, has sufficientinformation to make alpha nuclides known in itboth qualitatively and quantitatively.

Principles of the analysis and points of decisiveimportance in preparation of samples and in measure-ment are described.

Detectors

Detectors used in this experiment,were Si SBDs.The SBDs were made of high resistivity p-type Sisingle crystal( p>50 kohm-cm ). The radiationsensitive Al electrode was vacuum-evaporated on thefront face, and the periphery was protected by Epox2yresin. The sensitive area of the detector was 20 cm .

The radiation sensitive depth, which is bias dependent,could be extended to 2 mm or more by applyingsufficiently high bias voltage. The Al electrodeadheres strongly to Si that powder samples can bedirectly mounted on the surface electrode withoutdeteriorating the detector characteristics. The deepdepletion layer enables simultaneous measurement ofalpha- and beta-rays spectra.

The large area Si SBD was surrounded by anti-coincidence counters, which were consisted of plasticscintillation counters, and the detector assembly wasinstalled inside of 10 cm lead shield to lower thebackgrounds. Beta-ray spectra of environmental samplessuch as soil , -could be measured by this system.

Correlation measurement between alpha- and beta-rays and the analysis are now under study and to bepublished elsewhere.

The backgrounds in the energy region above 2 MeVcome mainly,(1) from alpha emitting U- and Th-series nuclides

contained in Epoxy resin as its impurities, and(2) from detector surface contamination by Rn and Tn

gases and their daughter nuclides while beingstored or in measurement.

Two ways were tried in setting samples withrespect to the detector; they were(1)powdered samples were directly mounted on the

Al electrode and(2)powdered samples were put in a sample holder,

and the detector was set on the holder withthe sensitive Al electrode downwards facingthe powder samples.

The measurements were made in air and the samples wereprepared to be infinitely thick for the alpha particleranges.

Energy Spectra of Alpha ParticlesEmitted from the Surfaceof Infinitely Thick Samples

Simple calculation shows that alpha energyspectrum emerging out of a plane surface of a

uniforml) distributed monochromatic alpha source, isgiven by

dn(E)/dE = (SA/4)(1/(dE/dx)) (1)

, whereS is the detector sensitive area in cm2,which is equal to the sample area, A is thealpha specific activity in dps/mg, anddE/dx is the stopping power of the medium inwhich alpha emitters are uniformly contained.

Formula (1) shows the observed alpha spectrum isinversely proportional to the dE/dx of the medium.

241Am solution was mixed with ultra fine Ni powderof a few hundred A in size and uniformly adhered onthe fine grains. The alpha energy spectrum from thethick source was measured by a SBD, and the results

0018-9499/86/0200-0639$01.00(1986 IEEE

639

agreed well with Formula (1) as shown in Fig.l.In deducing the Formula, uniform distribution of

alpha emitters and no serious scattering of alphaparticles in the medium are assumed. Fig.1 clearlydemonstrates that the scattering gives very littleeffect to the alpha spectrum in the energy regionabove 0.4 MeV.

241'Am in A1203

..

I .-

2 4

T.P.

.1 1'-_

6

ENERGY (MeV)

Fig.1 Alpha energy spectrum of 241Am, measured by p-SiSBD. Am was contained uniformly in fine powderof Al03, and directly mounted on the radiation

sensitive Al electrode.

2 4 6 8 (MeV)ENERGY

Fig.2 Alpha Energy spectra of ThO2 and ThO2 in Al 0respectively, measurement arrangement as 24?Amof Fig.l.

Alpha spectra from ThO2 powder and from ThO2mixed in A1203 are illustrated in Fig.2. In ThO2powder, the measured spectrum shows good agreementwith the calculated one by Formula (1), however in the

mixed source distinct deviation is observed as sharppeaks at the edges of the alpha spectrum. The edgescorrespond to respective alpha-ray energies at the

time of emission.This peaking phenomenon is caused by the

existence of comparatively large grains in Al20powder, which result in non-uniform distribution or

the alpha emitting nuclides in the medium. The

peakings were observed in several actual low activitysamples. One example was IAEA S-3 - a standard of

low grade Uranium ore. The alpha spectrum showed

slight peaking effect at the energy edges.

The edges in a continuous spectrum correspond to

the alpha energies of respective alpha emittingnuclides. Therefore, nuclides can be identified bydetermining the alpha energies at the respectiveedges.

Quantitative Analysis of Alpha Activities

Three kind of techniques, as shown in Fig.3, can

be applied to determine the respective alphaactivities. They are,

(1) total integration,(2) region of interest integration, and(3) differential curve fitting. Before applying

these techniques, nuclides must be identified throughthe spectrum edge energies.

(1) Total Integration TechniqueFormula (1) is integrated above several

discrimination levels. For example, in the case ofFig.3-(a), the following Formula can be formed;

N1=(S/4)(A1(R(E1)-R(e1)) + A2(R(E2)-R(e1))+ A3(R(E3)-R(e1)))

N2=(S/4)(A2(R(E2)-R(e2)) + A3(R(E3)-R(e3))

N3=(S/4) (A3(R(E3)-R(e3))

(2)-i

(2)-2

(2)-3

, whereN1 is the total counts of alpha pulsesexceeding a discrimination level e1,

E1, E2, and E3 are the alpha energiesof the respective nuclides, andR(E) is the alpha particle range ofenergy E in the medium. The parametersare also shown in Fig.3.

The Formula (2) can be formed as many as thenumber of the nuclides involved. So that intensity ofeach nuclide A1 can be determined by the linearequation system. In this technique (1), statisticalaccuracy is excellent , because all the data pointsabove the respective threshold levels are utilized.

This technique will introduce errors, however, ifpeaking phenomenon has been observed near the edges inthe measured spectrum.

(2) Region of Interest IntegrationThis technique (2) can get over the difficulty of

distortion or peaking in a measured spectrum. Formula(1) is integrated for three regions of interest asshown in Fig.3-(b).

N1 = (S/4)(A1+A2+A3)(R(e2)-R(e1))

N2 = (S/4)(A2+A3) (R(e4)-R(e3))

N3 = (S/4)A3(R(e6)-R(e5))

(3)-l

(3)-2

(3)-3

By solving the three linear equations, the inten-sitiy of each alpha emitting nuclide can be determined.

640

-

z

I-

-L%-07-

641

(3) Differential Curve FittingThis is an iteration technique. Nuclides and the

intensities are approximately assumed first. ApplyingFormula (1), a curve is calculated and compared withthe measured spectrum. The assumed values aresuccessively corrected to get a better fit.

-iw

zz

C)

z0~CoC-

(a)

\

CO)z

00N3

el e2 e3 _

e1 e2e3 e4e5 e6

(c)

E1 E2CHANNEL NO.

101-

10° _o

E3

Fig.3 Principle of determining alpha activities;(a) Total Integration Technique,(b)Region of Interest Integration Technique,(c) Differential Curve Fitting Technique.

Examples of Quantitative Analysis

(1) Effect of Different Sample Mounting MethodTwo mounting methods were tried as illustrated in

Fig.4,(A) a powder sample were directly mounted on the

sensitive electrode, and(B)the same sample was put into a holder, and the

alpha sensitive electrode faced to the sampledownwards, leaving an air gap of 1.5 mm.

The spectrum measured in the arrangement (A),was similar to the one calculated by Formula (1).While in (B), alpha energy was shifted and distortedby the air gap. In addition to the air effect,intense sharp peaks of Rn daughters were observed.

The findings can be understood as follows:Rn gas out of the sample remains in the air gap,successive alpha emission recoils the daughter nuclideand the recoiled atom adheres on the detectorelect

9 daughters of 235U series were more intense

than 222Rn daughters of 238U series in the spectra.The experimental results can be explained by thea!Sference in the half lives of two Rn nuclides.

Rn has much longer half life than 219Rn so that the

(A) SampleDet. t- '

2 4ENERC

0a.1 m X~~~~~~~k TP

_ ,.

[.

~~~10GY (MeV)

Fig.4 Alpha and beta energy spectra of Uranium ore;(a)sample was directly mounted on the electrode,(b)Al electrode faced downward to the sample.

gas diffuses away from the air gap before the decay.The peak intensity of Rn daughters depends upon

the sample form, the measuring time interval etc.,therefore, care must be taken to look at these peaksfor quantitative analysis. These peaks are to be usedfor qualitative analysis, as a measure of relativeintensity between U- and Th- series activities whenthey co-exist in environmental samples.

(2) Analysis of Commercial ProductsAs an example of applying this alpha activity

analysis method, Uranium concentration in a chemicalfertilizer was determined.

In phosphatic fertilizer production process,phosphoric acid is separated from phosphate rock. Itis clearly demonstrated in the alpha spectrum offertilizer( Fig.5 ) that Uranium has co-extracted withthe phosphoric acid in the chemical process.Quantitative analysis of Uranium through this3pectrum, applying Total Integration Technique, showedbhe fertilizer contained 150 ppm of natural Uranium.

TP

\ FERTILIZER

o I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~238u '

O X 1 ~~~234U

z -* ._ ..O. ___X_____ _._

GYPSUM BOARD

CH.NO.

Fig.5 Alpha and beta ene'rgy spectra of(a)Fertilizer, and (b) Gypsum board. Samples

were placed under the Al electrode.

-4

N

S,11

S,

The numerical figure agreed well with the value

?Qained by neutron activation analysis, usingU(n,gamma)239U nuclear reaction. Fig.5 also shows

alpha spectrum of gypsum board samples. The spectrumshows a part of U-series after 226Ra and thecontamination of Th-series as well. The gypsum boardhad been made of phosphate rock residue afterextracting phosphoric acid. The process can beconfirmed comparing two spectra in Fig.5.

(3) Analysis of Environmental SamplesAs examples of environmental samples, Fig.6 shows

alpha spectra of (1) granite standard sample-G2, (2)sea deposit near coast, and (3) deposit in swimmingpool which has not been in use for more than 10 years.

These samples were set under a Si SBD with theradiation sensitive electrode downward, so thatintense alpha peaks of Tn and An daughters were

observed in the energy region above 6 MeV. Smoothingprocess had been applied to the spectra. In eachsample, activities of U-series against Th-series can

be estimated relatively, comparing the alpha peakcounts of Tn and An daughters.

The hump on the high energy side of sharp 212Po

fflha peak, is caused by coincident beta pulses ofBi - parent of 212Po(T1=2- 3x10-7 sec.), whose

half life is so short that the beta pulses are summedup when injected to a SBD almost simultaneously withalpha particles.

In the pool deposit, contamination of 226Ra etc.from the wall paint were observed. Quantitative analy-sis of nuclides in the sample have not been attemptedyet. However, for the sample in which natural U- andTh-series are contained, contamination of artificialnuclides can easily be distinguished, if the changesin the spectra with time have been recorded.

C)w

0I)

to0

I-

z

0

)ck STD G-2 0_OTJ: 1.99 ppm C,

rh: 25.2 ppm

ibmarine Deposit .

The alpha spectra measured by a large area SBD

will greatly improve the sensitivity of contaminationdetection, compared with the conventional gross alphaactivity measurement. This spectral information is

useful for soil samples, in which the relativeintensity of U-series against Th-series varies with

the sampled location; and results in the fluctuationof the natural backgrounds. With the spectralinformation, one can understand more clearly the cause

of anomalous variation of alpha activities.

Conclusion

Simple and rapid quantitative analysis of alphanuclides is feasible by analyzing energy spectra ofalpha particles emerging out of infinitely thicksources. In applying this new method, it mus-t beconfirmed that alpha emitters in the samples are

distributed uniformly.The alpha spectra can be affected by the way of

mounting samples with respect to SBDs. For dis-torted alpha spectra, appropriate quantitativeanalysis technique should be selected.

The alpha spectra analysis described in thispaper, enables to improve the detection sensitivity ofcontamination due to artificial radio-activenuclides,easily distinguishing them from naturalbackground. Since this is non-destructive alphanuclide analysis, the method is suitable for thescreening of large number of samples. This new methodcan be effectively applied to contamination detectionin various samples at the time of nuclear accident as

well as the routine monitoring of artificial radio-activities in reactor facilities.

References

1) F.Shiraishi, Y.Takami, and M.Hosoe, " A Large Area

Solid State Detector Made of Ultra High Purity P-

Type Si ",Nucl. Instr. & Meth., 226(1984),107-1112) M.Hosoe, Y.Takami, F.ShiraishiandK.Tomura;

"Stopping Power Measurement Using Thick AlphaSourced', Nucl. Instr. & Meth.,223(1984),377-381

)Sit

..

4 6810

ENERGY (Me.V)

iFig.6 Energy Spectra of environmental samples with A1electrode downwards.

This work is partly supported by the Science ResearchPromotion Fund bf the Japan Private School PromotionFoundation.

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