Ion Chamber Primer

Description

It is impossible to completely describe all the intricacies of the detector behind the focal plane of the FMA, but this document should answer some basic questions. To know more you should talk to C.J. Lister.
The ion chamber at the back of the FMA has a segmented anode for energy measurements at high count rates. It has 3 segments along the beam direction and up to several segments in the other direction to reduce pile up at high count rates. The configuration is adjusted with jumpers inside the chamber. Standard charge-integrating preamplifiers and spectroscopy amplifiers are used to read out the anodes.
Position information is derived from delay line readout off the ends of the X wire plane which runs the length of the chamber from the front to the back. Special Daresbury preamps are used on the ends of the delay line.
A timing signal is derived from the cathode which provides poor time resolution.

Specifications


                    DE1     DE2            DE3
                 -------- ------- ----------------------

                X~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~X

                G.......................................G


                W                                       W
		(                                       )      
	beam	(	                                )
	---->	(                                       )
		(                                       )
		(                                       )
		(                                       )

                C---------------------------------------C

where W=window, C=cathode, X=Position Wires, G=Frisch Grid.
V(cath) = - 19 Volts/torr
V(anode) = + 8 Volts/torr
window thickness = ~200 ug/cm**2 mylar (C10 H8 O4)
P = 10-50 torr (isobutane)
F = 200 sccm
distance between anode and position plane = 1 cm
distance between position plane and grid = 1 cm
height = 7.5 cm (cathode to grid)
width = 15 cm
depth = 30 cm (DE1=5 cm, DE2=5 cm, DE3=20 cm)

Calibration

It is important to gain match the segments of the anode if you want to add the energies measured under each segment.

Last Updated: May 24, 1996 (D.J. Blumenthal, [email protected])