New Photonis XP20D0 photomultiplier for fast timing in nuclear medicine

Transcription

1 New Photonis XP20D0 photomultiplier for fast timing in nuclear medicine M. Moszyński, M. Gierlik, M. Kapusta, A. Nassalski, T. Szczęśniak, Soltan Institute for Nuclear Studies, PL Świerk-Otwock, Poland, Ch. Fontaine, P. Lavoute Photonis. Av. Roger Roncier, B.P. 520, F Brive La Gaillarde, France

2 Introduction First Time-of-Flight PET systems were developed in early eighties at LETI, Grenoble and Saint Louise, US, using CsF and later on BaF 2 crystals. However, a need for a high stopping power of crystals for gamma rays moved the interest to the conventional PET based on BGO. A success of TOF PET depends on both scintillators and photomultipliers. Within different crystals LSO and LaBr 3 are studied in this respect.

3 Introduction The aim of this work was to study the performance of a new Photonis XP20D0 with the enhanced timing capabilities. It is due to: - a screening grid at the anode - enhanced quantum efficiency of the photocathode The timing study were done for LSO and LaBr 3 crystals

4 PMTs in a fast timing A low transit time jitter A high number of photoelectrons A low contribution of time jitter in the dynode structure In the study done by B. Bengtson and M. M. (1979), a deterioration of a time resolution by a commonly used construction of the anode was discovered.

5 PMTs anode and screening grid The anode is built as a grid inside of the last dynode Induced current signal is due to movement of electrons from the last dynode, However, the other component of the signal is induced by electrons traveling from pre-ultimate to the last dynode.

6 Earlier study of the problem B. Bengtson and M. M. dynode timing in XP2020 and RCA8850, which improves time resolution by 1.2 factor, tested for fast plastics and NaI(Tl). Several other experiments and experimental arrangements used the dynode timing. In sixties the famous XP1021 was equipped with the screening grid at the anode to speed up an initial rise of the anode pulse.

7 Earlier study of the problem Philips (at present Photonis) built the experimental XP2020UR-M PMTs with the grid. Tests of NE111, NE213 and NaI(Tl) showed the improvement of time resolution by 1.15 factor Photonis built XP20Y0/DA, being a prototype of XP20D0. Results of the study presented at IEEE meeting in Portland showed improvement by Recently Photonis introduced on the market XP20D0, based on this principle.

8 Tested photomultipliers Parameters of PMT XP20Y0/DA No. 021 XP20D0 No XP20D0 No Photocathode bialcaly Blue sensitivity [µa/lmf] White sensitivity [µa/lm] Time jitter, σ [ps] ±13 260±13 220±13 No of dynodes 8

9 Scintillators Crystal Size [mm] Manufacturer λ pk [nm] Decay time [ns] Light output [ph/mev] LaBr Saint Gobain LSO CTI ±500

10 Timing system CFD Ortec 935, optimized for each combination of PMT and scintillator TPHC Ortec 457 Slow-fast arrangement Time spectra measured in relation to the truncated BaF 2 crystal coupled to the XP20Y0Q/DA. Its contribution to time resolution was 90±4 ps for 60 Co and 128±4 ps for 511 kev annihilation quanta ( 22 Na).

11 LaBr 3 gamma spectrometry Energy spectrum of gamma rays from a 22 Na source, measured with the LaBr 3 crystal Na 511 kev XP20D 0 with LaBr 3 Note a superior energy resolution of 3.65% for 511 kev annihilation quanta, N um berofcounts 6000 FW HM =3.65% kev C hannelnum ber For 662 kev peak, energy resolution of 3.15% was measured.

12 LaBr 3 timingstudy The 1.33 MeV peak selected in LaBr 3. The measured time resolution of 140±4 ps corresponds to 107±4 ps for LaBr 3 detector The 511 kev peak: time resolution of 200±4 ps corresponds to 154±5 ps for the LaBr 3 detector N um berofcounts Co XP20D 0 with LaBr 3 1ch=11.6 ps FW HM 140 ps 22 Na XP20D 0 with LaBr 3 1ch=11.6 ps FW HM 200 ps C hannelnum ber Phe number = 17600±500 phe/mev

13 LSO timing study The 1.33 MeV peak selected in LSO. The measured time resolution of 146±4 ps corresponds to 115±4 ps for LSO detector The 511 kev peak: time resolution of 210±4 ps corresponds to 166±5 ps for the LSO detector Phe number = 8100±240 phe/mev N um berofcounts Co XP20D 0 with LSO 1ch=11.6 ps FW HM 146 ps 22 Na XP20D 0 with LSO 1ch=11.6 ps FW HM 210 ps C hannelnum ber

14 Time resolution for 511 kev gammas Crystal /PMT Time resolution, δt [ps] Measured Tested detector N [phe/511 kev] δt N [ps phe] 10 3 LaBr 3 /PMT ±4 166±4 8200± ± ±4 159±4 8700± ± ±4 154±4 9000± ±0.5 LSO/PMT ±4 185±4 3500± ± ±4 172±4 3930± ± ±4 166±4 4140± ±0.4

15 A comparison with the earlier experiments XP20D0 TTS = 520 ps, 13.7 µa/lmf, screening grid R5320 TTS = 150 ps, 9 µa/lmf LaBr 3 154±5 ps LaBr ps, J. Glodo, et al. (2004) LSO 166±5 ps LSO 212 ps W.W. Moses (1999)

16 Discussion A qualitative comparison: different crystals, different adjustment of timing, XP20D0 is comparable or better than R A screening grid and a very high blue sensitivity of XP20D0 are more important, than the superior time jitter of R5320. A low time jitter is of the great importance for plastic scintillators.

17 Discussion The time resolution measured with LSO and other slow decaying scintillators, depends stronger on the statistic of photoelectrons produced in the decay process of the light pulse, while time jitter of the PMT has a weaker influence on measured time resolution. Predicted by the Hyman theory of timing (1965) Known from a study of NaI(Tl)

18 Conclusions The study showed the excellent timing capabilities of the XP20D0 PMTs. The importance of the screening grid at the anode and a superior quantum efficiency were pointed out. The excellent time resolution measured with LSO for 511 kev annihilation quanta suggests strongly that the XP20D0 can be used in the realistic project of TOF PET.