AT-TPC

Design of the proposed active target time projection chamber: a) installed within the solenoid; b) view of chamber with removable target wheel.

Active Target Time Projection Chamber (AT-TPC)

The AT-TPC combines time projection and active target functionality in a single device thus allowing measurements of rare processes that require high detection efficiency and large acceptance, and low energy processes that are traditionally difficult to measure due to the short range of the reaction products in matter. As the name implies, the AT-TPC will operate in two different modes. In the active target mode, the AT-TPC counter gas acts as both a target and detector, allowing investigations of fusion, isobaric analog states, cluster structure of light nuclei and transfer reactions to be conducted without significant loss in resolution due to the thickness of the target. The high efficiency and low threshold of the AT-TPC will allow investigations of fission and giant resonances with fast fragmentation rare isotope beams. Operating the AT-TPC in the detector mode, the reaction products created in collisions between isospin asymmetric heavy ions will allow the density dependence of the symmetry energy term of the nuclear equation of state to be explored. To accommodate this range in experimental programs the AT-TPC is designed to be portable to allow the chamber to be installed at a variety of NSCL beam lines, including the new reaccelerator area.

The detector consists of an active target-time projection chamber installed in an external magnetic field. The tracks, momenta and energy loss of charged particles passing through the detector volume will be measured to identify the particle species and properties. The infrastructure of the AT-TPC will be composed of a gas filled chamber with a planar pad readout array, gas mixing and laser calibration systems, and a large-bore, 2T solenoidal magnet. The AT-TPC chamber consists of a gas filled volume at pressures ranging from 0.2 to 1 atm contained within a stainless steel cylinder with a length of 120 cm and radius of 35 cm. The beam will enter the gas volume through a 1-2 cm diameter mylar window. The thickness of the window will be dependent upon the experimental requirements and can range from 3-20m. The readout plane will cover the surface area of the chamber end-cap including the entrance window region for the beam. The readout array consists of 10,000 pads with a size of the order of 0.5cm x 0.5cm each. Target gases of interest include H2, D2 and 3He. In the active target scenario, reactions may occur at any point along the beam path through the chamber. It is anticipated that the AT-TPC will be used with ancillary detectors to provide additional information about the characteristics of the collision events being studied. For this reason beam particles and reaction products may exit the detector through a 30 cm radius mylar window. The fine pad segmentation coupled with the magnetic field will allow intermediate energy protons and light particle species emitted in an angular domain greater than 5 degrees with respect to the beam to be measured with a momentum resolution better than 0.5%. Low energy particles will be stopped in the detector gas and will have correspondingly shorter tracks. The energy resolution from the tracking alone should be of the order of ~0.5%, as limited by straggling phenomena.

The AT-TPC is funded through the NSF MRI program and is a collaboration between researchers from MSU, University of Notre Dame, Western Michigan University, LLNL, LBNL, and St. Mary's University (Canada).