Cooling detectors in particle physics

1. Cooling detectors in particle physics Gavin Leithall CCLRC Rutherford Appleton Laboratory

2. CCLRC Rutherford Appleton Laboratory • Government funded central research laboratory which supports a wide range of university research activities • Located in Oxfordshire • I work in the Particle Physics Department on the vertex detector for the International Linear Collider Placement Conference 2006

3. The International Linear Collider • Will collide beams of electrons and positrons with energies from 91 - 500 GeV (upgrade to 1000 GeV) • Scheduled to begin operation in 2015 • Will have a total length of about 30 km • Intended to complement the Large Hadron Collider by being a more precise measuring tool • Together they are hoped to discover new particles and test theories (e.g. the Higgs Boson and Supersymmetry) Placement Conference 2006

4. What is a vertex detector? • Collisions produce a spray of high energy particles • A large detector is built around the beam pipe to work out what happened in the collision • The vertex detector is the one closest to the collision point • Used to reconstruct particle tracks to determine their production point (vertex) • Required to have little material to minimise scattering Placement Conference 2006

5. Linear Collider Flavour Identification • LCFI (my project group) is designing the vertex detector for the ILC • The detecting elements called ladders are layered in concentric barrels • The ‘hits’ generated when a particle passes through enable track reconstruction Placement Conference 2006

6. M N N “Classic CCD” Readout time  NM/Fout Column Parallel CCD Readout time = N/Fout Detector Technology • The main technology being developed by LCFI is the Column Parallel Charge Coupled Device (CPCCD) • These are composed of tiny pixels which accumulate charge when particles pass through • Similar to the CCDs in digital cameras, but with a much faster readout • The readout chips are placed at the end of the ladders Placement Conference 2006

7. Detector Cooling • The vertex detector will produce heat which will need to be removed. • It will also need to be maintained at a constant operating temperature (possibly as low as -70 deg C) • It therefore needs a cooling system to meet these requirements • Conventional cooling systems would add material to the detector volume, so are not ideal • Blowing cold gas from the ends of the detector is a possible solution • My project is to investigate the effectiveness of this Placement Conference 2006

8. Filter Heat exchanger Thermocouple Mass flow controller Gas Heater Regulator To Control Box Computer Liquid nitrogen Cooling Test Rig • Built a system to produce a controlled nitrogen gas flow with • Variable temperatures (-100oC to 20oC) • Variable flow rates (0-20 litres / min) • Built a system to read temperatures from platinum resistors • Designed programs to enable remote control of both of these systems Placement Conference 2006

9. Detector model • A quarter barrel model was decided upon because it would be easier to build than a full barrel, while maintaining all the essential physics. • It has: • Stainless steel ladders and aluminium end-plates • Resistors in the place of the readout chips to simulate heating • Platinum resistors at various positions within the quarter barrel Placement Conference 2006

10. Side view of quarter barrel Resistors End plate Gas In Gas Out Ladders End plates Quarter barrel construction Inlet Outlet Placement Conference 2006

11. The Physics Power lost to surroundings = Ps Surrounding temperature = Ts Quarter barrel Temperature = Tq Gas In Gas Out Temperature = To Flow rate = v Temperature = Ti Flow rate = v Heating power = Pi Placement Conference 2006

12. Formulation of the problem • Pg (power gain of gas) can be calculated by Pg = cv (To – Ti) (c = specific heat capacity) • Using energy conservation Pi = Ps+ Pg • Using Newton’s Law of Cooling Ps = L (Tq – Ts) (L = thermal loss coefficient) • A graph of (Pi - Pg) against Tq should • Be a straight line with gradient L • Pass through Pi – Pg = 0 when Tq = Ts Placement Conference 2006

13. This graph • Is a straight line with a gradient giving L ~ 0.26 W / deg C • Suggests a room temperature of Ts ~ 19 deg C Placement Conference 2006

14. A hypothesis • I can make a hypothesis about the form of Pg: Pg = hv (Tq – Ti) • h is the heat transfer coefficient, assumed constant, but could be a function of v, Tq, Ti • This can be tested by plotting graphs of Pg against the other variables Placement Conference 2006

15. This graph gives strong support to the hypothesis that Pg is proportional to (Tq – Ti) By plotting the gradient (i.e. Pg / (Tq – Ti)) of each line against its flow rate, the hypothesis can be tested further Placement Conference 2006

16. This graph supports the hypothesis that Pg / (Tq – Ti) is proportional to v The gradient of this graph gives h ~ 0.022 W / deg C / (litre/min) Placement Conference 2006

17. Conclusions • Thermal loss coefficient L ~ 0.26 W / deg C • The form of Pg is Pg = hv (Tq – Ti) • Heat transfer coefficient h ~ 0.022 W / deg C / (litre / min) • Maximum Pg~ 5 W when v = 20 litres / min and (To – Ti) ~ 11 deg C Placement Conference 2006

18. Summary • Testing the effectiveness of gaseous cooling for the vertex detector for the International Linear Collider • Results so far show behaviour that is consistent with predictions • Move on to investigate new configurations • More inlets, and with different positions • Different sizes and angles of inlets Placement Conference 2006

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21. Plotting the value of Pg predicted by the hypothesis against the value obtained by the earlier measurement provides a useful crosscheck This yields a graph which provides good support for the hypothesis Placement Conference 2006