Pulse height non-linearity in LaBr 3 :Ce crystal for gamma ray spectrometry and imaging

Transcription

1 Pulse height non-linearity in LaBr 3 :Ce crystal for gamma ray spectrometry and imaging P A O L O B E N N A T I I N F N R O M A T R E E D E M O M P H D S C H O O L, R O M A T R E U N I V E R S I T Y P A N I R., C I N T I M. N., P E L L E G R I N I R., R I D O L F I S., S C A F È R., M A T T I O L I M., O R S O L I N I C E N C E L L I V., D E N O T A R I S T E F A N I F., F A B B R I A., S A C C O D., N A V A R R I A F., L A N C O N E L L I N., M O S C H I N I G. A N D B O C C A C C I O P.

2 outline Measurement of pulse height distrubution of LaBr3:Ce coupled to different photomulplier tubes are presented. R6231 PMT shows pulse heigh non-linearity when coupled to LaBr3:Ce crystal. Energy Resolution of LaBr3:Ce crystal are measured with pulse height calibration. 12th SeminarIPRD / 06 / 2010 paolo.bennati@uniroma3.it

3 Relative Light Yield Scintillator specifications for SPET imaging Density (g/cm 3 ) 140keV (mm) Z eff. Photofraction (%) Light yield (ph/mev) Decay time (ns) ΔE/E FWHM (%) (@140keV) (PMT) λ Emiss max (nm) NaI:Tl , LaBr 3 :Ce ,000 Data are from Saint Gobain Crystal 16 (97%) W.Moses, NIM A, 487 (2002) 1,3 1,25 1,2 1,15 1,1 1,05 1 0,95 0,9 0,85 NaI(Tl) A LaBr 3 (Ce) B Luminosity 662keV) ER of LaBr3(Ce) close to the statistical limits 0, Energy (kev) A Energy (kev) Prescott and Narayan, NIM A, 75 (1969) B G.Bizarri, IEEE TNS, Vol 53,02 (2006) Constant Light Yield for E>30keV

4 LaBr 3 :Ce/PMT pulse height non-linearity P. Dorenbos et al., TNS, VOL. 51, NO. 3, JUNE 2004 Gamma Ray Spectroscopy With a Ø 19 x19 mm 3 LaBr3 :0.5% Ce3+ Scintillator HV=-700V Co 60 gamma ray pulse height spectra measured with LaBr 3 :Ce a)hv = -500 V b)hv = -700 V. HV=-500V LaBr3:Ce current yield, about 25 time higher than NaI:Tl crystal, is responsible of current saturation within dynodes 12 th Topical Seminar on Innovative Particles Radiation Detector Siena 7-10 / 06 / 2010 paolo.bennati@uniroma3.it

5 Scintillation crystals investigated LaBr 3 (Ce) cylinder, 12 mm Ø 12 mm thickness Reference for spectrometry LaBr 3 (Ce) slab, mm 3 + (3 mm glass window) NaI(Tl) slab, mm 3 + (3 mm glass window) Optimized for SPET imaging 12 th Topical Seminar on Innovative Particles Radiation Detector Siena 7-10 / 06 / 2010 paolo.bennati@uniroma3.it

6 Photomultiplier Tubes investigated PMTs # Dynode stages Quantum efficiency (@380 nm) Size (mm 2 ) Dynode structure Gain Typ (x10 6 ) Multianodes Voltage Divider Investigated HV supply range R6231 Standard PMT % 51 Linear Focused 0.27 No Modified (ID. AS20 S.Gobain) V R7600U % 25 x 25 TABLE II PMT parameters Metal channel 2.0 No Tapered (ID. E5996 Hamamatsu) Tapered & V R7600U % UBA 25 x 25 Metal channel 1.0 No No-tapered (ID. E5996 Hamamatsu) V H8500-C % 49 x 49 Metal channel x8 Array standard Hamamatsu divider V H8500C- MOD % SBA 49 x 49 Metal channel x8 array standard Hamamatsu divider V All the PMTs are from by Hamamatsu (Japan)

7 Pulse height analysis with LaBr 3 :Ce cylinder Example: Ba 133 Pulse height distributions 31 (Cs ka) 81 kev R6231 Standard PMT kev kev Spectrometric read-out electronics: - Pre-amplifier -Custom Amplifier (shaping time=1ms) -ADC Ortec 916 buffered PC card (12 BIT) sofware MAESTRO32 31 kev (Cs ka) R UBA Radionuclides used for test Am kev 356 kev kev Co 57 Ba 133 Cs 137 Na 22 Co 60 Bibl. NNDC National Nuclear data center. Brookhaven National Laboraty, Upton NY USA 12 th Topical Seminar on Innovative Particles Radiation Detector Siena 7-10 / 06 / 2010 paolo.bennati@uniroma3.it

8 Energy resolution results (with LaBr 3 :Ce cylinder crystal) Energy (KeV) ER (%FWHM) R6231 (standard PMT) ER (%FWHM) R UBA Better energy resolution results for R6231 at low energies Some unexpected issues

9 Spectrum Anatomy: Energy alignment procedure R6231 standard PMT: Ba 133 Pulse height distribution of LaBr 3 :Ce cylinder TWO step procedure: 31 kev R6231 Standard PMT 10 k = 0.65 kev/ch 1) The four emission-lines of Ba133 are used to define the calibration coefficient [energy range kev] 81 kev kev 2) We add an offset, if needed. 384 kev IMP: Each spectrum is INDEPENDENTLY calibrated

10 Spectrum comparative analysis - LaBr3:Ce cylinder 7 Misalignment! Perfectly overlapped After calibration, we overlap the spectra: A) Some differences are clearly visible at low energies range. B) R6231 shows non-linearity effect at energy lower than 100keV : true measured 31 kev >> 45 kev ~35 % 53 kev >> 60 kev ~15 % 81 kev >> 85 kev ~5 % C) The new R PMT shows a superior response in the whole energy range. 12 th Topical Seminar on Innovative Particles Radiation Detector Siena 7-10 / 06 / 2010 paolo.bennati@uniroma3.it

11 R (UBA): Energy spectrum (w and w/o tapered divider ) R UBA: Ba133 pulse height spectra with LaBr 3 :Ce cylinder crystal 31 kev 81 kev R w/o tapered divider shows 10% of nonlinearity at E<81keV 276/303 kev 356/384 kev R with tapered divider does not show PH nonlinearity in the whole HV range investigated Tapered dynodes stages ratio: 2:2:2:1:1:1:1:2:3:2. Un-tapered dynodes stages ratio: 1.5:1.5:1.5:1.5:1.5:1:1:1:1:1:1.

12 H8500 MA-PMT : Spectrum analysis with mm 3 slab crystals 1) LaBr3(ce) and NaI(Tl) coupled to MAPMT H8500 (BA photocatode) Anodes are connected as a short circuit 5 276/303 kev 356/384 kev MA-PMT H8500 coupled with LaBr3:Ce shows a linear pulse height with respect to energy Measurements confirm the wellknow behaviour of NaI(Tl) at low energy (high light yield) 2) MA-PMT H8500-Mod (SBA photocatode) + LaBr3:Ce 5 MA-PMT H8500 is linear with energy, but the new H8500-mod SBA presents non-linearity (20% at 31 kev)

13 Peak by peak energy calibration evaluation Pulse Height distributions of LaBr3:Ce cylinder crystal: +35% This evaluation confirm R6231 standard PMT pulse height non-linearity at E<100keV The new R UBA presents a superior response in term of pulse height linearities.

14 Peak by peak calibration for 50x50 mm 2 slab crystals The values of k are normalized to H8500 ones MA-PMT H8500 response with LaBr3:Ce is linear with energy. The new H8500-mod SBA presents non-liearity (10-20)% at E<81 kev MA-PMT H8500 measurements confirm NaI:Tl light yield is higher than LaBr3:Ce at low photon energy. [E 662 kev, range of interest of Nuclear Medicine]

15 Energy Resolution of LaBr 3 :Ce cylinder with pulse height non-linearity calibration Energy (kev) ER (%FWHM) R6231 Standard PMT ER (%FWHM) R UBA ± ± ± ± ± ± ± ± ± ± 0.07 * Theoretical limit = a/ N phe ± ± 0.05

16 Energy Resolution of SLAB CRYSTALs with pulse height non-linearity correction Energy (kev) ER (%FWHM) R6231 Standard PMT ER (%FWHM) MA-PMT H8500 SBA ± ± 0.9 * Theoretical limit = a/ N phe ± ± ± ± ± ± ± ± ± ± 0.2

17 Conclusions R6231 PMT shows pulse height non-linearity at E<100 kev. PMTs with metal channel technology dynodes present superior response in pulse height. H8500-mod shows 10-20% non-linearity effect for E<70-80keV. ER of LaBr3:Ce crystal is proportional to 1/radq(N phe ) After correction, LaBr shows ER values better than NaI(Tl) at photon energy higher than 40 kev. UBA and SBA PMTs do not present ER values as expected from higher QE, probably due to the lower charge collection efficiency of 1 st dynodes.

18 thank you Paolo Bennati INFN Roma3 EDEMOM PhD school, Roma TRE University Rome Italy

19 Cs137 Energy spectra 32keV (Ba ka) kev (backscatter) 477 kev (Compton scatter) 662 kev Cs137 Energy spectra of LaBr 3 :Ce cylinder with R6231 and R7600 UBA PMTs after calibration