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Response Function of a 3×3 in. NaI Scintillation Detector in the rangeof 0.081 to 4.438 MeV

Hashem Miri Hakimabad, Hamed Panjeh* and Alireza Vejdani-NoghreiyanPhysics Department, Faculty of Science,Ferdowsi University of Mashhad,Mashhad, Iran&FUM Radiation Detection And Measurement Laboratory,Ferdowsi University of Mashhad, Iran

Abstract : Response functions of a 3×3 in. NaI detector, which is mainly used in PGNAAapplications, have been calculated by using MCNP-4C code. Calculated results are comparedwith measured data by using standard γ-ray sources to check their accuracy. γ-rays fromradioisotope sources were used in the range from 0.081 to 4.438 MeV for this determination.Through the precise modeling of the detector structure, the agreement between both resultshas been improved.

Key words : FWHM; GEB; NaI (Tl); MCNP code; Monte Carlo simulation; Nonlinear response;Response function; PGNAA

Asian J. Exp. Sci., Vol. 21, No. 2, 2007, 233-237

IntroductionUp to now, many experiments and

theoretical analyses have been done to matchsimulation results and experiment responsesobtained by NaI (Tl) detectors (Hadizadehet al., 2004; Zhang and Gardner, 2004; Guoet al., 2004; Gardner et al., 2000; Gardnerand Sood, 2004; Mitra and Sarkar, 2005;Vitorelli et al., 2005). Also, the importantfeature of nonlinearity of NaI(Tl) detectorsfor PGNAA (Prompt Gamma NeutronActivation Analysis) applications wasinvestigated with great detail by Gardner(Gardner, 1999).

NaI detectors are still used frequently inindustrial PGNAA applications. They havethe advantages that are efficient for high-energy γ-rays, they are rugged, and they canbe used without cooling.

In the present paper Response functionsof a 3×3 in. NaI scintillation detector wereestimated using Monte Carlo simulations withthe MCNP code (Briesmeister, 2000). A newapproach was used and the accuracy of thecalculated responses was verified for somespectra through comparison with experiment.

Experimental ApproachExperimental setup which is used in this

study is to take γ -ray spectrum fromstandard γ-ray sources. The experimentalmethod for γ-rays from radioisotope sourcesdoesn't need a complex setup. To obtainrelatively good spectra, the NaI detector islocated 15 cm from the γ -ray source in a"low scatter - low background" room, 80 ×80 × 80 cm, whose walls are 10 cm thick.The inner sides of the walls are lined with0.03 in. Cd sheet and 0.015 in. Cu sheet inthat order.

* Corresponding author : Hamed Panjeh, Physics Department, Faculty of Science, Ferdowsi Universityof Mashhad, Mashhad, Iran

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When using NaI detectors, andconsequently photomultiplier tubes (PMTs),the quality of the data can drasticallydeteriorate through spectral smearing due totemperature and/or counting rate changes.While counting rate changes directly affectthe gain of the PMT, temperature changesaffect the whole detection system, includingamplifiers and high-voltage supplies (Knoll,2000). So the long-term stability of thedetection system is essential to maintainingthe high quality of the experimental data thatis obtained (Metwally and Gardner, 2004)especially in the PGNAA method.

Detector Specification and MonteCarlo Simulation

The NaI (Tl) detector used to obtain allspectra in this work is contained in a thinwalled aluminum can (0.005 in. wallthickness) to reduce absorption of low energyphotons and to prevent excessive Comptonscattering from the packaging material. Theoptical reflector is a 0.005 in. thick sprayedcoating of α -alumina. The crystal is mounteddirectly on the face of an RCA 8054 electronmultiplier, optically coupled with silicongrease (Dow QC-2-0057), and the entireassembly evacuated. The radioisotope-basedPGNAA setup and the standard γ -rayspectroscopy setup were modeled with greatdetail in Monte Carlo simulations. Thedetector structure was modeled as preciselyas possible, except details ofphotomultipliers.

The initial responses of the MCNPcalculation (pulse height tally, F8) werebroadened with the GEB option. GaussianEnergy Broadening (GEB) is a specialtreatment for tallies, to better simulate aphysical radiation detector in which energypeaks exhibit Gaussian energy broadening.

GEB is called by entering FTn card in theinput file of MCNP. The tallied energy isbroadened by sampling from the Gaussian:

20

)(⎟⎠⎞

⎜⎝⎛ −−

= A

EE

CeEf

where,E = the broadened energy;E0 = the unbroadened energy of the

tally;C = a normalization constant andA = the Gaussian width.

the Gaussian width (Valentine, 1996) isrelated to the Full Width at Half Maximum(FWHM) by :

2ln2

FWHMA =

the desired FWHM that is specified bythe user-provided constants, a, b, and c,shows a nonlinear response:

2cEEbaFWHM ++=

where E is the incident γ -ray energy.

The FWHM is defined as

)(2 0EEFWHM FWHM −= where

EFWHM is such that:

)(2

1)( 0EfEf FWHM =

and f (E0)is the maximum value of f (E).

Fifteen standard γ -ray sources in therange from 0.081 MeV to 4.438 MeV, Table1, were used to determine a, b, and c asparameters specify the Full Width at HalfMaximum in the GEB option. A program

Hakimabad et al. (2007) Asian J. Exp. Sci., 21(2), 233-237

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based on least-square approach was appliedto calculate the amount of a, b, and c basedon the presented FWHM formula andexperimental FWHM and resolutions listedin Table 1. According to this work the authoruses the following parameters in the GEBoption to generate detector responses,Figures 1, 2.

α=-0.00789 MeV; b=0.06769 MeV1/2;c=0.21159 MeV-1.

Results and DiscussionFor standard γ -ray sources, the

directional response functions were measuredand compared with the simulated results. Bymaking the detector more precise step bystep a good agreement (relative deviation lessthan 3%) has reached up in the whole energyrange. Figure 1 shows a good agreement inthe energy range of two photoelectric peaksand Compton edge for the 60Co gamma-ray

Response Function of a 3×3 in. NaI Scintillation Detector

Nuclide Eγ(MeV) Resolution (%)166Ho 0.081 16.19177Lu 0.113 13.5133Te 0.159 11.5177Lu 0.208 10.9203Hg 0.279 10.1451Cr 0.320 9.89188Au 0.411 9.217Be 0.478 8.62137Cs 0.661 7.754Mn 0.835 7.26207Bi 1.067 6.5665Zn 1.114 6.2922Na 1.277 6.0788Y 1.850 5.45Am-Be 4.438 4.28

Table 1

Fig.1. Comparison between simulation and experimental gamma-ray spectrum ofthe 60Co point source.

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spectrum. While the discrepancy in the lowerenergy region would be mainly due to thecontribution of gamma-rays scattered fromthe experimental Photomultiplier.

A bare 241Am-Be neutron source withγ -ray component (4.438 MeV (Croft,1989)) by itself was one of the fifteenstandards γ -ray sources. Figure 2 showsthree peaks, full energy peak, single escapeand double escape, of the experimental241Am-Be γ -ray spectrum which arecompletely matched with the simulated result.Between standard gamma sources spectraacquisition just this spectrum was acquiredin a free space where the 241Am-Be and NaIdetector were completely far from anyinterfering material to prevent excessiveCompton scattering or prompt gamma-raysdue to (n, γ) interactions. The source waslocated 3 m above the ground level and 1mfrom the center of the detector.

In order to simulate the total responseof a "bare" 241Am-Be neutron source due toγ-ray component of the 241Am-Be (4.438MeV) and those prompt gamma-rays thatare generated by the neutron interaction withthe source capsule and detector, twoseparate input files were used and the finalresult is the summation of these two part.One is to track gamma-rays in the detectorvolume which originate from the sourceposition and the other is to track promptgamma-rays arising the neutron interaction bythe capsule and detector materials.

ConclusionThis work has presented a way to

simulate the response functions of NaI (Tl)by using GEB as a special treatment fortallies in MCNP-4C.

Results show that MCNP simulationsby using GEB, fit all the Gaussian peaksarising from standard gamma-ray sources in

Hakimabad et al. (2007) Asian J. Exp. Sci., 21(2), 233-237

Fig.2. Comparison between simulation and experimental gamma-ray spectrum ofthe 241Am-Be neutron source (capsule format X.14).

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a wide range of energy from 0.81 to 4.438MeV, typically 60Co spectrum and 241Am-Be spectrum were shown. Also thesecoefficients a, b and c as described inSection 3, can be used by GEB option togenerate valid elemental library spectra forPGNAA analysis. Note that GEBparameters are different for eachconfiguration of experimental setup.

We have calculated nonlinear responsefunction of a NaI Scintillation Detector Byusing the MCNP code and checked theiraccuracy by comparing with the measuredresponses for standard γ -ray sources. Thecomparison results have shown that in the60Co spectrum the calculated response agreewith the measured one, less than ±3%relative deviation (corresponding to σ).

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