A CdTe detector with a Gd converter for thermal neutron detection

Transcription

1 A CdTe detector with a Gd converter for thermal neutron detection Aki Miyake, Takahiro Nishika, Shailendra Singh, Hisashi Morii, Hidenori Minura, Toru Aoki. Nuclear Instruments and Methods in Physics Research A 654 (2011) Paper review 12/12/11 Edward Cazalas

2 Objectives of Paper Create thermal neutron detector for neutron imaging with... High resolution Fast response time Room temperature operation. Thermal neutron detection through measurement of gamma energy in (n,g) reaction. Previous work done by group includes use of CdTe semiconductor for neutron detection Semiconductors have fast response times and operate at room temperature. Higher energy resolution is one purpose of this work

3 Objectives of Paper Modify use of CdTe semiconductor for neutron detection. Want high energy resolution to determine gammas generated in detector vs. those created in-beam (background) Obtain high energy resolution through application of higher electric fields in semiconductor make CdTe thinner. Use Gadolinium converter on top of CdTe to increase neutron attenuation lost by thinner CdTe.

4 Why Cadmium, Gadolinium. Thermal neutron (n,g) reaction Cadmium: ~12% 113 Cd (σ = 2.06E4 b) Tellurium: ~0.9% 123Te (σ = 370 b) Gadolinium: ~15% 155Gd (σ = 6.1E4 b), ~16% 155Gd (σ = 2.55E5 b) On average natural Gd has cross section 20 times higher than Cd How thick should Gd converter layer be? Use GEANT4

5 Detector construction CdTe is 4 x 4 x 0.5 mm Gd converter is 4 x 4 x? mm Gd converter is pasted with silver onto gold electrode Operation at 200V (why? Presume this voltage of max linear e- collection) Preamplifier (shaping time set at 0.5 us) Pulse height analyzer Assume indium electrode is negatively biased relative to the gold electrode. What about charge created in Gd converter?

6 Simulation of Gd Converter GEANT4 used to optimize Gd converter thickness to allow maximum photon collection by CdTe Assumed thermal neutron at ev Random distribution (xy position) beam source simulated neutrons Max at 25 um thick Gd

7 Potential issues with Simulation of Cd Converter Why simulated neutrons? Typically we only care about statistics. Plot should be gamma ray per neutron Error bars on plot... These are statistical means and need standard errors. Gd thickness... What about CdTe thickness? Are we stuck with a single thickness for a reason?

8 Simulation of Resolution Determine spatial resolution Infinitely narrow beam source centered in the middle of Gd converted Divide accumulated counts into 50 x 50 um pixels. Commercial CdTe pixel size = 100 x 100 um. Results give spread of 50 um one pixel Point spread function for 25 um Gd converter thickness

9 Simulation of Resolution Results give spread of 50 um one pixel FWHM of point spread function

10 Issues with Simulation of Resolution This plot does not portray very much information. Perhaps plot several peaks corresponding to several Gd converter thicknesses. Again, statistics What was the thickness of Gd converter? Not obvious.

11 Issues with Simulation of Resolution Why 50 x 50 um pixel size? What about smaller thickness Gd converters? Even a single data point below the selection point would be very useful in characterizing resolution behavior vs. Gd thickness

12 Build detector Irradiate Gd converter thickness = 25 um 241 Am = 59.5 kev gamma 57 Co = kev gamma 59.5 kev = 3.4 kev kev = 3.7 kev

13 Build detector Irradiate 252 Cf source (1.74 Mbq) surrounded by polyethylene moderator to thermalize emitted neutrons. Also surrounded by Lead and Tin to attenuate gammas (n,g) reaction prompt gammas = 157 Gd 80, 182 kev 155 Gd 199 kev 113 Cd 95 kev X-rays from Gd conversion electrons

14 Build detector Irradiate Spectrum background contains Compton events and background gammas from (n,g) reaction with lead, tin, polyethylene. It is claimed that neutron events are separable from background gammas. Technically true, however by the nature of the experiment, background gammas would not be expected to be strong.

15 Thoughts Would be nice to have quick calculation cross checking the simulated results to the experiment (could calculate total number of counts of all Gd(n,g) and Cd(n,g) gammas in spectrum and compare to estimated number of incident gammas in simulation). Would be interesting to see effectiveness of different design (wider CdTe, or Gd converter with CdTe on both sides, for example. Would likely shift optimum Gd converter to much larger thickness thereby increasing attenuation of neutrons without serious spacial losses of gammas). Simple to do in simulation. A simulation studying the response as a function of CdTe thickness (correlated with Gd converter thickness) would be valuable.