Gaseous Detector Laboratory


Reduction of Anode Wire Modulation


A method of construction of multiwire chambers has been developed which yields x-ray images with very small non-linearity, in particular that due to anode wire modulation. The technique requires an absorption region in the detector, with the anode and upper cathode wires, which are parallel to one another, in registration with each other. Studies show that the strength of the electric field in the drift region determines the degree of anode wire modulation and, under specific conditions, modulation is halved in periodicity and significantly reduced in amplitude. New interpolating cathodes have been developed to read out position in the other axis, along the anode wire direction. Position resolution between 100-150 m FWHM in both axes has been achieved. Two-dimensional images of small objects have been taken which show unprecedented linearity.


The two x-ray transmission images below show the dramatic improvement in the position linearity of the detector response across the wire direction.  The image on the left was taken from a 2D detector with no optimization.  The image on the right was taken from the detector with optimized wire arrangement.  



Two key factors contribute to the improvements in the image quality.  First is to position the cathode wires directly aligned with the anode wires, and tune the drift field to achieve the best linearity across the anode wires.  The second factor is the small wire pitch used in this detector (0.58mm).  The figure below shows a set of uniform irradiation responses (top) from a detector under different drift fields, and the corresponding calculated electron drift line plots (bottom).



Reference:
G. Smith and B. Yu, "The Linearity Performance of a Two-Dimensional, X-ray Proportional Chamber with 0.58 mm Anode Wire Spacing,"  IEEE Trans. Nuc. Sci. NS-42 (1995) 541-547.



Last Modified: Wednesday, 06-Feb-2013 22:33:56 EST