Electrical design considerations for a 40 MHz gas ionization chamber

The front IR quadrupole absorbers (TAS) and the IR neutral particle absorbers (TAN) in the high luminosity insertions of the Large Hadron Collider (LHC) each absorb approximately 1.8 TeV of forward collision products on average per pp interaction (~235 W at design luminosity 10³⁴ cm^-2 s^-1). This secondary particle flux can be exploited to provide a useful storage ring operations tool for optimization of luminosity. A novel segmented, multi-gap, pressurized gas ionization chambers is being developed for sampling the energy deposited near the maxima of the hadronic/electromagnetic showers in these absorbers. The ionization chamber must be capable of resolving individual bunch crossings at 40 MHz. The ionization chamber is segmented into quadrants; each quadrant consists of sixty (40×40) mm² Cu plates 1.0 mm thick, with 0.5 mm gaps. The 0.5 mm gap width has been chosen so that the time for the ionization electrons to drift across the gap, is short enough to produce at the output of the shaping amplifier, a signal that returns to the base line is less than the 25 ns bunch spacing of the LHC. From noise considerations in the presence of a cable the stack of plates are connected electrically 10 in parallel, 6 in series to achieve an equivalent detector capacitance C_d~50 pF. This type connection forms an electrode inductive L_e and electrode capacitive C_e network that must be optimized to transfer charge from the chamber to the sensing amplifier. This paper describes the design of the collection electrodes optimized for 40 MHz operation