A Scintillating Fiber Tracker for Cosmic Muon Tomography

Transcription

1 A Scintillating Fiber Tracker for Cosmic Muon Tomography Carsten Mai I. Physikalisches Institut B der RWTH Aachen GK Seminar Bad Honnef, September 2011

2 Outline A Particle Physicist's Approach to Archeological Problems Tomography by Means of Cosmic Ray Muons The Scintillating Fiber Tracker Recent Developments Geant4 Simulation Results A Tomograph Prototype Design of the Muontomograph Conclusions / Outlook 2

3 A Particle Physicist's Approach to Archeological Problems 3 pyramids of Giza Largest: Cheops Two other Pyramids have about the same size Search for hidden chambers till 1970s 19th century: dynamiting pyramids in order to find new chambers 3

4 Known structure of Cheops and Chephren differed Images taken from: Search for Hidden Chambers in the Pyramids, Louis W. Alvarez A Particle Physicist's Approach to Archeological Problems Cheops Pyramid Is the difference to be explained by epochal changes or is there a (very well) hidden chamber? Solution: Use omnipresent particle source to scan the composition: Chephren Pyramid Cosmic Rays 4

5 Images taken from: Search for Hidden Chambers in the Pyramids, Louis W. Alvarez Chephren Pyramid Belzoni chamber is the only access 5 planes of spark chambers/scintillation counters ±45 field of view Alvarez: Measurement of cosmic ray intensity from within the Pyramid 5

6 Intensity Profile of known King's chamber N W E (arbitrary sky coordinates) S 6

7 Monte Carlo Prediction for solid rock N W E E S 7

8 Measurement of Chephren Pyramid N 8

9 No chamber visible N 9

10 Discussion of the Result Archeologists: No Chamber has been found Alvarez: It has been proved, that there is no chamber 10

11 Pros and Cons no artificial radiation source is needed a reliable measurement result prevented archaeologists from time consuming searches low spatial resolution Installation of spark chambers inside a pyramide is not trivial non-destructive mean (no need to dynamite the pyramid) 11

12 Enhancement of Alvarez Idea Measure small angle scattering instead of intensity Benefit: correlation with atomic number θ0 = MeV Z x / X 0 [ ln x / X 0 ] βcp θ0 15 βcp L Lrad Drawback: necessity of measuring incoming and outgoing track 12

13 Distinguishing different Materials Scattering Angle RMS / 1 radiation Length 13

14 Point of closest approach Tracker layers Tracker layers above and below the target Measurement of incoming / outgoing angle Scanned Volume is divided in Voxels/Pixels 14

15 Point of closest approach Extend the tracks into the target volume Determination of Voxels either crossed by incoming or outgoing track Tracker layers 15

16 Point of closest approach Tracker layers Assign scattering angle to the voxel 16

17 The Scintillating Fiber Tracker 250 µm Photodetector Strip Width 17 17

18 The Scintillating Fiber Tracker Tracker Module two ribbons made of 5x μm thick scintillating fibers mounted on Rohacell foam/carbon fiber support structure 1 stereo angle between fibers read out with 32 channel SiPM arrays 50 μm single point resolution 18 18

19 Why to use a SciFi Tracker? Large scale area Detector is possible Active parts are kept outside the trigger acceptance Noise is only proportional to number of channels instead of the instrumented area 19

20 Attenuation Length of SCSF-78MJ Optical Attenuation length is composed of two parts 20

21 New 128 ch SiPM Array Old 32 channel SiPM Array was replaced by 128 channel Array Reduced epoxy layer covering active area 96 pixels / ch. (80 pixels before) Consists of two 64 ch. arrays 21

22 Temperature Stabilization Bias voltage can be passively corrected for temperature drifts If operating temperature is well known Passive regulation has been considered, but was deactivated 22

23 Spiroc Chip based Readout Electronics 256 ch./frontend board VA_32/75 chip replaced by new SPIROC chip for pulse shaping/amplification Channel-wise bias voltage control 23

24 The new Muon Tomograph 24

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26 MC Proof of Concept Geant4 Detector simulation Possibility to study potential of distinguishing Materials Concrete (solid block) selected for visualization from iron (font) via cut 26

27 First Muon Data Placed a 50 mm steel sphere between 4 tracker double layers, 32 cm² sr acc. 2 days of data at a target acceptance of about 8.5 cm²sr 45 cm 27

28 Geometrical Acceptance of Target Trigger TARGET Roughly every fourth track crosses the Target. Trigger: 32 cm²sr, Target: 8.5 cm²sr 28

29 First Muon Data 29

30 First Muon Data 30

31 First Muon Data Eye guides only! 31

32 First Muon Data 32

33 Applications Missing steel reinforcement of concrete leads to serious structural problems Construction company saves money Muon Tomography can serve non-destructive material testing 33

34 Applications Trace undeclared radiactive material even through huge layers of lead Geometric acceptance of std. Truck: ~70 m²s Scannable to the level of dm³ within a few hours =>fast detection of hazardous candidates 34

35 Conclusion / Outlook Already achieved visualisation of simple objects with MC and data Investigate more complex algorithms Gather more statistics with protype Development of the SciFi tracker goes on Mass production of readout electronics starting this month 35

36 Thank you very much for your attention! 36

37 Time-of-Flight System TOF module Bicron BC-408 scintillator bar 6 x 50 x 395 mm3 (λ = 430 nm) two modules at top and two at bottom scintillator bars optically separated with reflective aluminized Mylar film 2 optical hybrids 8 Hamamatsu S C on each side of module SiPM calibration LED read out with NINO discriminator chip digital temperature HPTDC-based sensor board digitizer 37 37

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