Influence of lead impurity on the scintillation performance of NaI(Tl) crystals

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PERFORMANCE OF NaI(Tl) CRYSTALS G SCHULZ and G BREITER

Zweltes Phyvtkah~ches lnstltut tier Umversltat Heidelberg Recewed 9 August 1967 Pb + centers in NaI(TI) absorb the fluorescence radtatton, thus deteriorating the energy resolution of big crystal sapprecmbly This effect ~s studted quantltatwely

For efficient ~,-detectlon big NaI(TI) crystals are most suitable 1) However, the energy resolution of big crystals is severely harmed by impurities that have absorption bands in the wavelength region of the fluoresence radiation 2) Because of these impurities the fluorescence photons are differently absorbed, depending on the crystal length they have transversed before impinging on the photocathode Thus, a monoenergetic y-radiation is detected with bad energy resolution when it is absorbed in different parts of the crystal Lead ions seem to be the most detrimental impurity They can neither be removed by crystallizing N a I from an aqueous solution 3) nor by zone melting techniques 4) and they have an absorption band at a wavelength near the m a x i m u m of the fluorescence spectrum The Pb + absorption band in NaI(T1) crystals was measured for several Pb concentrations by using an apparatus described earher s) The wavelength at band maximum was determined to be (3590__ 10)A This result agrees with the value given by Harshaw et al 2) For the fwhm we found (150___ 5)A. The Pb concentration was measured by a tracer method applying the

c m -1

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radioactive isotope 210pb The variation of the absorption coefficient at band maximum with Pb concentration is depicted in fig 1. It can be described by a straight line Inserting the slope of this straight line into the generalized Smakula formula6), we determined the oscillator strength of the Pb + center to be f=009+001 Knowing the oscillator strength and the absorption spectrum of the Pb + center as well as the fluorescence spectrum of NaI(T1)[ref 7)-Iand the spectral response of the photomultlpher*, we were able to calculate an absorption coefficient for the fluorescence radiation due to the Pb impurity # = 0 04___001 c m - I per p p m Pb This result is derived for # d < 0 2 ( d = a b s o r p t l o n length), which IS the region of technical interest. In a second experiment we measured this absorption coefficient directly using the arrangement shown in fig. 2 Freshly cleaved crystals with cross sections of 1 cm × 1 cm and thicknesses d were entirely surrounded by silicon oil t and mounted on a photomultlpher* Through a small, centered aperture they were irradiated with Kr K X-rays from a 83Rb source The mean free path of Kr K X-rays in NaI(T1) is 0 005 cm Therefore, the fluorescence radiation is produced only * RCA 8054, spectral response S-11 t Dow Cornmg Corporation, DC 200/200 000

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,~ 30 O u

,- 20 9 o 10

~ , d f_f~l I ~

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10 20 30 ppm Pb c o n c e n t r a t i o n

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83Rb source

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l e a d aperture slh con oil NaI(.TD crystal photomultlpher

Fig 2 Arrangement for the measurement of the absorption coefficient for fluorescence radmUon

F~g 1 Varmt[on of the maximum absorption coeffioent of the Pb + absorphon band in NaI(T1) with the Pb concentration

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G SCHULZ

AND G BREITER

at the upper surface of the crystal, and d 1s approximately the absorption length for the fluorescence photons With this arrangement we measured the pulse height distributions of various crystals with thicknesses between 0 02 cm and 1 0 cm and Pb concentrations of 5 p p m and 10 ppm From the decrease of the pulse height of the X-ray peak with increasing crystal thickness we derived the absorptlon coefficxents for the fluorescence radlaUon at these Pb concentrations Since the reflection propemes vary with crystal thickness, all measurements were done relative to pure NaI(TI) crystals (Pb concentration less than 0 1 ppm) Averaging the absorption coefficients for 5 ppm and 10 ppm Pb, we found / ~ = 0 0 3 _+001 cm -~ per ppm Pb, which agrees within the hmlts of error with the value given above in order to demonstrate the influence of Pb impurity on scmtlllatmn performance, let us consider an 8"dla x 8" hgt NaI(TI) crystal containing 0 05 ppm Pb Irradiating ~t with 1 84 MeV ),-quanta from the decay of 88y, we measure an energy resolution AE/E of about 8°0, instead of 4 5°0 which is normal for smallcrystals s) This increase is due to the mhomogeneous attenuation of the fluorescence radiation which vanes from 0°o to 3 5°o depending on the crystal length the photons have to transverse It can be partially compensated by

modifying the reflection properties of the crystal surface But, Iowermg the Pb contents would be more efficient. Using very thin crystals ( d = 0 0 2 c m ) with the arrangement shown m fig 2, we ascertained that the Pb + centers do not only absorb the produced fluorescence light but also lower the production rate of this radiation This quenching effect decreases the light output by 3% per ppm Pb Since the decrease is always homogeneous 4) throughout the crystal, it does not markedly affect the energy resolution We are mdebted to Harshaw Chemle N V (Netherlands) for providing us with Na! raw material References 1~ D A h a g a - K e l l y a n d D R Nicoll, Nucl lnstr a n d Meth 43 (19661 110 -') J A H a r s h a w , H C K r e m e r s , E C Stewart, E K W a r b u r t o n a n d J O Hay, NYO-1577 (Techn l n f Service, O a k Ridge, 19521 3) O H a h n , H K a d m g a n d R M u m b r a u e r , Z Krl~t 87 (19341 387 ~) G Schulz, Nucl In~tr a n d M e t h (19671 to be p u b h s h e d ') H Leutz a n d G Schulz, Z Physlk 192 (1966) 299 fJ) D L Dexter, Sohd State Physics 6 ( 1 9 5 8 ) 353 7) W J van Solver, Nucleonics 14 (19561 50 ~1 H Leutz, G Schulz and k van Gelderen, Nucl Meth 40 (19661 257

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