Imaging chambers in medicine, biology and astrophysics F.A.F. Fraga LIP - Coimbra, CFRM and Departamento de Física da Universidade de Coimbra, Coimbra, Portugal

Outline Imaging gas scintillators The GEM - an active scintillator CCDs Apllications –Quality control –Imaging chamber Alpha tracking Neutrons –Radiography –Therapeutic beam monitoring Other projects –Neutron spectrometer –Thermal neutron imaging –Pollarimeter Conclusions

Introduction 2D imaging detectors Advantages of optical readout –Electronics decoupled from detection media –Insensitive to electronic noise or RF pickup signals –Real multi hit capability with true pixel readouts - complex events –Large areas without dead spaces - optical systems (lenses, mirrors, fibers and tapers) New developments in optical imaging detectors, A. Breskin, NIM A498(1989)457c-468c

Gaseous avalanche chambers with optical readout 2D gas scintillators with optical readout by PMs or intensified CCDs Initially used with wires and pure gases –Xe, Kr, Ar and He with the addition of N 2 - UVscintillation, innefficient and expensive optics, optical wavelenght shifters Improvements –continuous amplifying structures (PPAC, grids) –gas mixtures scintillating at > 250 nm The gas proportional scintillation chamber, A.J.P. Policarpo, Space Sci. Instr. 3(1977)77

A few examples High pressure xenon (up to 20 bar) waveshifter fibers –A. Parsons, B. Sadoulet, S. Weiss, T. Edberg, J. Wilkerson, IEEE TNS 36(1989) Multistep, low pressure and high gas gain (light gap ~ 9 mm, light yield up to 3 ph./el.) –A. Breskin, R. Chechik and D. Sauvage NIM A286(1990) PPACs at atmospheric preassure light gap ~ 1.5 mm, TEA, TMAE, Penning effect, higher light to charge ratio –G. Charpak, W. Dominik, J.P. Fabre, J. Gaudaen, F. Sauli and M. Suzuki, NIM A269(1988) –V. Peskov, G. Charpak, W. Dominik and F. Sauli NIM A227(1989) TPC with optical readout (multistep, low pressure TEA) –U. Titt, A. Breskin, R. Chechik, V. Dangendorf, H. Schmidt-Böcking and H. Schuhmacher, NIM A416 (1998)85 Optical imaging with capillary plate, argon-TMA and intensified CCD. –T. Masuda, H. Sakurai, Y. Inoue, S. Gunji and K. Asamura, IEEE TNS 49(2002)

Some limiting features Low number of emitted photons –image intensifiers - expensive, degrade image resolution, limited size Large scintillation gaps –degrade position resolution, diffusion, optical depth of field Technically complicated and expensive –low pressure, high temperature, capillary plates

Luminiscence in microstrips 1993 A. Oed and P. Geltenbort reported high luminosity from pure gas mixtures 1998 We used scintillation to perform quality control of microstrips –CCD with Ar2% Xe Microstrip operation in noble gases: an active scintillator, P. Geltenbort and A. Oed, Proceedings of the Workshop on Progress in Gaseous Microstrip Proportional Chambers, Grenoble, June 1993 Towards a method for quality control of microstructures for gaseous detectors based on scintillation light, F.A.F. Fraga, M.M. Fraga, R. Ferreira Marques, J.R. Gonçalo, E. Antunes, C. Bueno and A.J.P.L Policarpo

The GEM should be a good candidate for a gas scintillator See F.Sauli. NIMA386(1997)351

Electric field simulation Magnitude of the electric field along the center of the GEM channel for equal measured gain in GEMs of different metal hole size Thin gap, high gain, no blurring

Study of luminiscence of GEMs Both charge and light signals were digitized

Typical light signal shape using He- 40%CF 4 The light signal risetime at the preamplifier output is 39ns

Average rise time of the light and charge signals versus induction field, E i 55 Fe E d =0.5kV/cm E t = E i = 2kV/cm double GEM gain ~ 3.1x10 3.

Energy resolution

CCD characteristics CCD camera: QUANTIX 1400 (PHOTOMETRICS) Number of pixels 1317 x 1035 (6.8 x 6.8 mm pixels) Read noise (1 MP/s) 18 e RMS Dark current 0.03 e/p/s (-25ºC, Peltier cooled) Binning - 2x2 up to 7x7 – less position resolution but lower noise! Nikon 50mm f1.8 photographic lens with C mount adapter Quantum efficiency of the Quantix 1400 camera versus wavelengh

What is a CCD? Pixel type silicon light sensitive detector High quantum efficiency - up to 90% - but no gain Integrating type device - exposure time from ms to minutes Limited range Low noise - cooling can be needed Pixel sizes up to 30 x 30  m High number of pixels up to 4000 x 4000 Analog-digital serial readout - slow

Why using CCDs for the readout of radiation detectors? High resolution - up to 4000x4000 pixels Large area detection using lenses or mirrors Can be placed away of detection media Cheap cost Electrical noise free Simple interface with computers

CCD readout of GEM scintillation Radiation source Minimum focusing distance~30cm

First images of GEM scintillation Scintillation image of a GEM foil. The holes of the GEM are seen as emitting dots in the small zone which is shown magnified Ar-2%Xe

Gas study and optimization Quality control Increasing the CO 2 amount lowers the light emission A small amount of quencher enhances stability of light emission Ar-5%CO 2 was found to be the optimum mixture for q.c. Light yield ~ 0.03 photons/secondary electron

Quality control –scintillation is sensitive to electric field configuration –checks GEMs gain uniformity –identification of local defects –finds optically unseen deffects

GEM characteristics Electrical field can have higher values than in PPACs Cascaded GEMs –Micro-Pattern Gaseous Detectors, by F. Sauli and A. Sharma, Ann. Rev.Nucl.Part.Sci 49(1999)341 High gain up to 4 stages, gain up to –J. Va´vra, A. Sharma, NIM A Vienna 2001 –A. Breskin, PSD6 Free from ion feedback –Study of ion feedback in multi-GEM structures, A. Bondar, A. Buzulutskov, L. Shekhtman, A. Vasiljev, 2002, submitted to NIM A Photon screening, free from photon feedback –R. Chechik et al. NIMA419(1998)423 Large areas (~30 x 30cm) –Gem detectors for COMPASS, by B. Ketzer, S. Bachmann, M. Capeáns, M. Deutel, J. Friedrich, S. Kappler, I. Konorov, A. Placci, K. Reisinger, L. Ropelewski, L. Shekhtman, F. Sauli. IEEEE NSS Lyon, No need to collect the electrons on the induction electrode avoiding breakdown in the last stage

Tracking chamber Sensitive volume ~250 cm 3 Track lenghts up to 8cm Cascaded standard double GEM (10x10cm) 30 cm

Tracking chamber views

Data on Ar CF 4 gain and relative luminosity E C =0; Ar 5%CO2 shown for comparison Ar CF 4 has greater light emission than Ar CO 2 Good light emission for higher percentage of quencher Ar-5% CF 4 light yield 0.57 photon/secondary electron (>400 nm) Performance of a tracking device based on the GEM scintillation, F. A. F. Fraga, S. T. G. Fetal, L. M. S. Margato, R. Ferreira Marques and A. J. P. L. Policarpo, Presented at the IEEE 2000 NSS

Visible and NIR emission spectra of Ar- CF 4 mixtures, normalized to the light intensity at 620 nm. Nº of photons emitted, between 400 and 1000 nm, per secondary electron, as a function of the effective gain, in Ar-CF 4 mixtures. (Measurements performed with the photodiode). The GEM scintillation in He-CF 4, Ar-CF 4, Ar-TEA and Xe-TEA mixtures, M. M. Fraga, F. A. F. Fraga, S. T. G. Fetal, L. M. S. Margato, R. Ferreira Marques and A. J. P. L. Policarpo, presented at Beaune 2002 conference, submitted to NIM A

Images of alpha tracks taken using the tracking chamber with Ar -5%CF 4 VGEM1=VGEM2=400V (Gain~140), ET =5.45KV/cm, EC=5.86KV/cm, Texp.=10ms. (a,b)VGEM1=VGEM2=400V (Gain~140), ET=5.45KV/cm, EC=5.86KV/cm, CCD Binning 4x4, Texp.=10ms; (c,d) VGEM1=VGEM2=430V (Gain~300), ET=5.45KV/cm, EC=0, CCD Binning 7x7, T=10ms.

Bragg curves of 241 Am alpha particles Light callibration using full tracks ~ 180 detected photons per deposited keV light yield ~0.6 photons/secondary electron

Projections of alpha tracks Ar-5%CF 4 Triple GEM, VGEM=450V, g=82, ED=1kV/cm, ET=3.4 kV/cm, b=7x7, EC=0, 241 Am alpha particles energy = 5.48 Mev Range of 241Am alpha particles in Ar = 3.42 cm The length and orientation of the track can be measured using charge or PMT signals Perfomance of a Tracking Device Based on the GEM Scintillation", F.A.F. Fraga, L.M.S. Margato, S. T. G. Fetal, R. Ferreira Marques and A.J.P.L Policarpo, IEEE Trans. on Nucl. Sci. 49, NO.1, February 2002, pg

3 He thermal neutron detectors Thermal neutron capture in 3 He 3 He + n  p + 3 H keV proton range = 4.4 mm, triton range = 1.6mm (1bar CF 4 ) R.B. Knott, G.C. Smith, G. Watt, J.W. Boldemann, NIM A392(1997)62

Data on charge gain and light emission in CF 4 pressures 400mbar, 1, 2 and 3 bar Gain saturates for smaller holes at lower pressures as reported in NIMA 419(1998)  m hole GEMs have higher light yield

Data on CF 4 + He CF 4 pressure = 400mbar, He = 0.6 and 3.6 bar Photon yield photons/secondary electron at 1 bar He-60%CF 4

Closed detector Clean GEM chamber- stainless steel –GEMs 5 x 5cm –50mm diameter transparent window –carbon fiber window or aluminum cover

Details of the clean GEM chamber

Images of proton and triton tracks in 3 He- 400 mbar CF 4 Triple GEM camera two 80  m, one 60  m metal hole absorbtion space 3 mm ED (drift field) =1KV/cm, ET (transfer field) = 3.25 kV/cm, EC (collection field) = 0 VGEM1 =VGEM2 =350V. Binning 7x7 AmBe source with Polyethylene shielding

Images of proton and triton tracks in 3 He- 400 mbar CF 4 Projection of the light intensity along the track as measured by the CCD CCD readout of GEM based neutron detectors, F.A.F. Fraga, L.M.S. Margato, S. T. G. Fetal, M.M.F.R. Fraga, R. Ferreira Marques, A.J.P.L Policarpo, B. Guerard, A. Oed, G. Manzini and T. van Vuure, Nucl. Instr. and Meth. In Physics Research A 478 (2002) 357

X-rays radiography Car key (~5 cm) radiography X-ray energy ~8keV Xe-10%CO2 at 1bar absobtion length ~3 mm Plastic gearwheel ~1.5 cm radiography Imaging detectors based on the GEM scintillation light, F.A.F. Fraga, L.M.S. Margato, S. T. G. Fetal, I. Ivaniouchenkov,, R. Ferreira Marques, A.J.P.L Policarpo, presented at the IEEE NSS 1999

High pressure Xe X-ray detector A 25 mm thick conversion volume at 5 bar Xe will have ~ 90% detection efficiency for 17.5 keV X-rays! 50 mm will be needed to get 80% efficency at 25 keV Performance of high pressure Xe/TMA in GEMs for neutron and X-ray detection, R. Kreuger, C. W. E. van Eijk, F. A. F. Fraga, M. M. Fraga, S. T. G. Fetal, R. W. Hollander, L. M. S. Margato, T. L. van Vuure, presented at the IEEE NSS 2001

High pressure Xe / TMA Xe-TMA strong Penning effect TMA ion. pot 8.1 eV Xe metastable pot. 8.3 Operation at a lower voltage single GEM 60/70/140

Operation of Xe - TMA at 5 bar (light) Light yield ~ 0.3 ph/ sec. electron ~10 4 ph / keV with gas gain 700 Scintillators have ph/ keV

UV CCD system ~40 keuro, CCD chip ~10 keuro

Radiography of a small dog-whelk double GEM, 5mm absorption space, Xe-2.5%TMA at 5bar, molybdenium X-ray tube at 40 kV

Radiography of a small snail ~8mm double GEM, 5mm absorption space, Xe-2.5%TMA at 5bar, molybdenium X-ray tube at 30 and 40 kV The width of the shell fissure is similar to the GEM picth

Images of a 50 micron slit collimator X-ray voltage 30kV Collimator length 25 mm Collimator slit 50  m  ~ 65  m CCD readout of high pressure xenon-TMA GEM detectors for X-ray imaging, L. M. S. Margato , F. A. F. Fraga *, M. M. F. R. Fraga *, S. T. G. Fetal *, R. Ferreira Marques *, A. J. P. L. Policarpo *, T.L. van Vuure , R. Kreuger , C.W.E. van Eijk  and R.W. Hollander , presented at the SAMBA 2002, Trieste, 2002

Xe-2.5%TMA rise-time at 1 and 3 bar versus collection field

Energy resolution using Xe- 2.5% TMA 5bar (light signals)

Recoil detector for fast neutron (1-10 MeV) spectroscopy Single event energy resolution Efficiency is expected to be more than two orders of magnitude better than current Li foil detectors (~10 -7 ) Gaseous media GEM multiplication Scintillation read by CCD

Recoil neutron spectrometer We have to measure –Energy of the recoil Total light measurement –Angle of the recoil nucleus ratio between track real length (estimated from the recoil energy) and projection read by the CCD

Gas selection (neutron recoil spectrometer) Maximal track length should be around 5 cm Efficient scintillator Experimental measurements with alpha particles are being carried on to estimate the accuracy of the spectrometer Tests will be done at the Democritos (Greece) neutron accelerator facility

Medical applications Dose imaging in radioteraphy Dose imaging in radiotherapy with an Ar-CF 4 filled scintillating GEM,S. Fetal, C.W.E. van Eijk, F. Fraga, J. de Haas, R. Kreuger, T.L. van Vuure and J.M. Schippers, PSD6, submitted to NIM

Other projects Thermal neutron imaging –Solid converter detector with GEM active scintillator readout –Groups integrating the TECHNI collaboration X-ray polarization –GEM polarimeter with optical readout GEM coating with p-terphenyl

Conclusions Active scintillators using GEMs can be used with a large variety of gases Ar-CO2, Ar-CF4, Ar-TEA, He- CF4, Xe-CO2, Xe-TEA, Xe-TMA,... Very large number of emitted photons per detected event –typically 2-3 orders of magnitude than solid scintillators Can achieve high resolution Large area Fast signals, high count rates(>10 5 c/s/mm 2 ) promising with APDs and position sensitive PMTs

Acknowledgements Current work on GEM luminiscence is supported by the contract CERN/P/FIS/ 2001/2567 with the Portuguese FCT. This work was done with the collaboration of the GDD, CERN(F.Sauli), TUD (C. van Eijk) and SDN, ILL (B. Guerard)

Photon-counting position sensitive devices APD arrays Hamamatsu S8550