TACTIC: A time projection ionization chamber for low energy nuclear astrophysics

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We propose the construction of a detector for the measurement of low energy charged particle reactions covering nearly the full solid angle. The detector, named TACTIC, will be a cylindrical time projection ionization chamber with the electrical field in radial direction. The central region will contain the gas target which, depending on configuration, can also be the detection gas in the field region. The central region is at constant potential and is not visible to detection. Ions, typically emanating from some nuclear reaction with a non-zero azimuthal angle, pass through a grid of wires leaving the central region.They will then produce a trace in the field region, i.e. free electrons.The electrons are drawn towards the anode of the electric field. However, on their way the electrons will encounter a GEM (Gas Electron Multiplier) foil amplifying the signal by a large factor. Signals registered at the anode strip will be digitized and later processed in an VME crate. Therefore the chamber will register full traces which include directions, a full energy measurement, and for each anode strip an energy loss measurement.In addition, a time correlation with the beam RF is possible. An array of BGO detectors will surround the chamber and allow to filter out gamma charged particle reactions. The anode strip will be divided in polar direction to allow for higher counting rates as well as for the detection of multiple particles. The reactions so far proposed include a measurement of the 8Li(alpha,n)11B reaction, the measurement of the 7Be(p,p)7Be elastic and inelastic reaction, and a 12C+12C experiment, with more to follow. We have built a test facility to observe signal amplification by the GEM as well as to test several gases in the chamber. All those tests have proven that the principle on which TACTIC is based is indeed sound and the construction of the chamber feasible.In addition, we have performed extensive GEANT calculations confirming basic design configurations. A similar detector, BONUS, has been built at the J-Laboratory and has been experimentally demonstrated to work.In this grant we request the funding for the necessary manpower and operational expenses. A separate RTI grant request submitted deals with the acquisition of the hardware.