The detector system of the EPOS system

Transcription

1 The detector system of the EPOS system 1. The detector arrangement 2. The lifetime system 3. Digital Doppler measurement 4. AMOC Martin-Luther-Universität RK Halle R

2 Detector system 3 experiments: lifetime spectroscopy (16 BaF 2 detectors); Doppler coincidence (2 Ge detectors), and AMOC (1 Ge and 1 BaF 2 detector) digital detection system: - lifetime: almost nothing to adjust; time scale exactly the same for all detectors; easy realization of coincidence - Doppler: better energy resolution and pile-up rejection expected; easy coincidence setup

3 t rep =77 ns repetition time is standard operation mode at ELBE is used for FEL s is perfect for lifetimes τ < 7 ns, i.e. most materials for nano-porous materials: t rep too short slow mode with t rep > 500 ns necessary real advantage of electron LINAC primary time structure can be adopted with low loss of overall intensity Beam repetition time for lifetime spectroscopy Counts Simulation parameters Statistics: 10 7 FWHM: 0,2 ns Background: 0,04% Channel width: 0,1 ns MC-simulated spectrum: lifetimes: 0.15 ns 2 ns 140 ns Intensity: 5 % 10 % 85 % Time (ps) thumb rule: τ max = 0.1 t rep Repetition time: C E G I

4 Lifetime system lifetime will be measured with direct digitizing of anode pulses external coincidence system is required to avoid useless digitizing (not shown) one of eight parallel lifetime channels

5 Lifetime system problem with mixing into one channel: delay cable must be elongated for slow mode several choices for the detector tubes: XP2020, HH , R7400U-096, and also the MCP-PMT s (previous talk)

6 Selection of PMT Philips Type photocath. diameter (mm) window range (nm) peak λ (nm) quant. eff. voltage (V) gain rise time (ns) transit time (ns) TTS (ps) cost (EUR) XP2020Q Head-on BA 51.0 H Head-on BA 51.0 R7400U-096 Metal package Cs-Te 11.0 fused silica fused silica fused silica R3809U-57 MCP-PMT Cs-Te 11.0 MgF ~ ~ EPOS-02 (J. Cizek)

7 XP 2020 Digital lifetime spectrometer Sweep of two anode signals XP 2020 too slow for a positron pulse of σ t < 100 ps (TTS 250 ps) recorded with 2 GS/s

8 Hamamatsu H successfully used e.g. at Tokyo University by H. Saito obtained resolution of 110 ps in coincidence setup (only 50 counts/s) not very stable in long-term use two tubes available in Halle in May 2004 we ll get 2 Photek MCP- PMTs for testing -> we will compare all detectors for final decision photo taken at Tokyo University

9 Anode spectrum of Hamamatsu R7400U recorded with 2 GS/s Anode pulse of ultra fast Hamamatsu R4700U-09 as measured with 2 GS/s digitizer spectral sensitivity fits best for BaF2: sensitivity for slow component (310 nm) reduced by 0.10 compared to fast one (220 nm) a faster digitizer is required (>= 4 GS/s) however: very small window (only 10 mm opening)

10 Cs-Te + silica photocathode R 7400U-09 fast slow EPOS-02 (J. Cizek)

11 Photek PMT 325 geometrical problem when arranging 16 detectors in a ring

12 Amplifiers necessary expected pulse height of single-stage PMT s: 1 mv however: expected lifetime > 10 a of continuous operation amplification necessary (40 60 db) easily done for f T > 5 GHz amplifiers also necessary to decouple the anode pulses when mixed together to one coincidence channel otherwise the anode pulses are intermixed and the coincidence circuit cannot work

13 Digital Doppler measurement sample rate can be smaller MHz resolution should be 14 bit (16384 channels) when 511 kev is at 80% of maximum energy -> 39 ev/ch only one such digitizer available: Compuscope (GaGe) is PCI-Card with dual input (single input: 100 MS/s and dual input: 50 MS/s) one card with additional memory is available in Halle (27 k )

14 Doppler coincidence easy to realize single channel Doppler and coincidence mode easy to realize by software hope: better energy resolution, higher throughput better time resolution expected more accurate detection of pile-up pulses -> lower background at E > 511 kev which is only reason for high momentum background in a 511 kev system Amplitude (mv) Channel A Channel B Time (µs) recorded with 50 MS/s and 12 bit resolution

15 Channel B Channel A Amplitude (mv) Time (ns)

16 Line-shape discrimination possible when additional disturbance of signal -> pulse will be ignored example additional RF of about 1 MHz overlayed only pulses with certain shape can be selected Amplitude (mv) Channel B Channel A Time (ns) recorded with 50 MS/s and 12 bit resolution

17 AMOC combination of lifetime and Doppler spectroscopy test needed -> can both cards operated in the same PC external coincidence required

18 Conclusions Digital detector for the whole EPOS possible Main advantages: - hardly anything to adjust - extreme stability - easy remote control - time scale for all detectors exactly known and equal within line-shape discrimination of pulses possible drawback: pulse rate of a single detector limited by online software processing of pulses to about 5x10 4 s -1

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