Doppler correction using segmented
Germanium detector
G.Turk1, A.J.Boston1, J.R.Cresswell1, S.Gros1, A.R.Mather1,
B. McGuirk1, P.J.Nolan1, J.Norman1, J. Sampson1, J.Thornhill1, C.Santos2,
J.Simpson2, I. Lazarus2, C.J.Hall2, W.I. Helsby3, R.A. Lewis4
1Department of Physics, University of Liverpool, Liverpool, UK.
2CCLRC Daresburg Laboratory, Warrington, Cheshire, UK.
3IReS Strasbourg, Strasbourg, France.
4School of Physics and Material Engineering, Monash University, Melbourne, Australia.
129 mm
43 mm
Doppler correction
The Doppler effect is described by the following formula:
E = E0(1+v/c cos(q))
where v/c is 5.6 % and q is the angle between the beam and the correcting element of the detector. For the incorrected data, the angle is 90 degrees. The correction is done for different parts of the detector. Showed is first the initial spectrum. This is taken from the raw data. The two other spectra show an improvement in energy resolution if a correction by sector and segment are performed. The resolutions of the peak are shown with the figure.
The 129 mm distance cannot be corrected as well as the closer distance. After the sector correction it is not improving. The smaller distance however can be improved with the segment correction. Hence the correction within a segment, using pulse shape analysis, can only be beneficial here.
31.0 keV
19.6 keV
19.6 keV
66.8 keV
26.5 keV
23.5 keV
Doppler correction
Pulse shape analysis
The next step in improving the resolution is utilizing the pulse shape analysis. It involves looking at the shape of each pulse.
A scan across the front face of the detector has been performed, resulting in a good characterization of the detector. An intensity plot can be made and show the non-coaxial electric field at the front of the detector very well. The risetime response for different parts of the detector are indentified. T30, T60 and T90 risetimes are calculated on an event by event basis. The risetime distributions as a function of radius are shown. A linear fit has been performed on the distribution for all the segments. For a given risetime the corresponding radius can be calculated.
Further work is required to Doppler correct the data with the pulse shape analysis.
T90
T30
High Purity Germanium detector have been used in gamma ray spectroscopy for years. The key aspect that makes them particular suitable is the excellent energy resolution. The last 5 years the development of the Ge-detectors moved on towards segmented germanium detector. These kind of detectors have the Germanium crystal electricly segmented into several individual parts. The segmentation improves the position resolution and makes them suitable for pulse shape analysis. By digitisation of the pulse shape a more precise interaction position can be achieved. Using this technique a better Doppler correction can be performed.
An experiment has been done at the university of Cologne using the tandem accelarator. A 70 MeV 37Cl beam incidents on a deuterated Ti-foil target of 500 mg/cm2 thickness with Cu backing of 200 mg/cm2. This induces the reactions:
D(37Cl, p)38Cl
D(37Cl, p)38Ar
The main interest goes out to the 2+ to 0+ which produces a 2167 keV gamma ray. The v/c of the recoils is 5.6% which should be sufficient for the Doppler correction.
Equipment and setup
Electric field lines
Holes
Electrons
A 24-fold segmented segmented detector is used in the experiment. The detector has a 80 mm long high purity n-type crystal with a diameter of 65 mm. The detector is electrically segmented into 4 rings and 6 sectors. Each segment has an individual outer contact and a common center contact.
The detector was placed at two different distances from the target, i.e. 43 mm and 129 mm. There is a large difference in energy resolution of the 2167 keV peak for the 2 distances. The difference in energy resolution is a result from this.
129 mm
43 mm
Eu 152 source
The data for the detector was digitized by a specially designed data-aquisition system. It consists out of 7 GRT4 VME modules. Each module has 4 input channels each consisting out of an amplifier and 14 bit 80 Mhz flash ADC.
Introduction
Intensity plots of the two rings of the detector
Risetime distribution
Email: gt@ns.ph.liv.ac.uk