Monocrystalline diamond films with several metal contacts were characterized as radiation detectors in terms of responsivity as a function of X-ray energy, uniformity of response as a function of the position, speed of response, and stability with the time. The responsivity as a function of X-ray energy was found consistent with the physics of the metal to diamond interface and with the characteristic of the crystal in terms of lifetime of charge carriers The uniformity of response as a function of the position was found to be within 10%. The speed of response was found to vary as a function of the metal contact. Finally, overnight exposures proved that the detector build is stable. The responsivity of one device as a function of energy was calibrated and such a device was used for plasma diagnostic at Joint European Torus (JET).
Figure 1: Schematic of the detectors built and of the electrical connections.
Very high quality monocrystalline diamond films suitable to build detectors have being grown for a few years [1]. Such crystals can be exploited effectively as X-ray beam intensity monitor because of their radiation hardness and capacity to whithstand a high termal load. The detector group of Diamond has been studying devices built by University of Rome “Tor Vergata” for a few years. Previous tests at the Synchrotron Radiation Source (SRS) in Daresbury resulted in very promising performance of the devices to be exploited as X-ray beam intensity monitor [2].
The work performed at the beam line B16 of Diamond Light Source aimed at testing more completely than in the previous test at SRS new devices provided by the University of Rome, it aimed at calibrating a device to be used for plasma diagnostic at JET, and at investigating the characteristics of different metal contacts. The versatility of the beamline B16 enabled us to scan easily the energy of the beam . Moreover the availability of the focused beam enabled to scan the surface of the detectors to study their uniformity. Neither of these characteristics were available during the previous test at SRS.
The characteristic to be tested were the linearity of the devices as a function of photon flux, their time response, their uniformity across the surface, and the stability of the responsivity with long exposures (over eight hours).
Figure 2: Photocurrent as a function of the beam position.
The samples prepared were monocrystalline diamond films grown on a high temperature high pressure diamond substrate with the first layer heavily doped with boron to act as a back contact. The front contact (a circular electrode with a diameter of 3 mm) was built by evaporating metals on the surface (Fig. 1). A few samples were prepared with aluminium contacts. In addition, four different metal contacts were built on the same substrate (Al, Pt, Au, Cr) to study their influence on the responsivity. The four contacts were built on the same substrate so as to evaluate the influence of the contacts only, rather than possible variation between different films.
The samples were placed in an earthed metal box so as to minimise the pick-up noise and operated in current mode with a FEMTO amplifier used as front-end electronics. The metal box was placed in a high precision translational stage that enabled the precision positioning of the detector with respect to the beam. The beamline ‘s data acquisition system was used for the steady-state and slowly-varying signals, whereas an oscilloscope was used to acquire the transient of the photocurrent. The photon flux was monitored by the drain current generated by the slits and by an ionization chamber upstream of the detector.
A monochromatic beam was used in the region from 4 keV to 20 keV. The beam was focused with the beamline optics (spot size about 300 mm) and could be completely intercepted by the detector. For the operation at low energy a tube filled with helium was used to minimise the air path, reducing the absorption. A scintillator was used to determine the harmonic content of the beam. The crystal was detuned during the calibration of the detector to be used at JET so as to minimise the harmonic content.
The responsivity as a function of X-ray energy was found consistent with the physics of the metal to diamond interface and with the characteristic of the crystal in terms of lifetime of charge carriers. With the help of Monte Carlo calculation and considering the charge diffusion in the undepleted zone of the device (zero electric fiels) the lifetime of the electrons was estimated to be 1.5 ns that is consistent with what is reported in the literature. These results helped in better understanding the physics of the device [3]. The responsivity of one device as a function of energy was calibrated and such a device was used for plasma diagnostic at JET [4].
Figure 3: Response to a transient depending on the metal contact.
In addition the uniformity of response as a function of the beam position was measured by scanning the beam across the device surface. The responsivity was found to be constant within 10% (Fig. 2). The speed of response was switching on and off the beam with the shutter of the beam line. The transient behaviour of the photocurrent was found to vary as a function of the metal contact (Fig. 3). This depends on the physics of the interface. However, time did not permit further investigation of this phenomenon, which will be revisited in the future. Finally, two overnight exposures confirmed that the detectors are stable.
This work demonstrated that the technology of diamond detectors is mature to be used in X-ray beam diagnostic. Further investigations have to be carried out to better understand the physics of the devices. An experiment with a structured device is planned at Diamond, which may find application as a position-sensitive detector.
References
[1] J. Isberg et al., Science, 297 ,1670 (2002).
[2] N. Tartoni, M. Angelone, M. Pillon, et al., IEEE Trans. Nucl. Sci. Vol. 59, No 3, (2009).
[3] S. Almaviva, M. Marinelli, E. Milani, et al., Journal of Apllied Physics, 107, 014511 (2010).
[4] M. Angelone, M. Pillon, M. Marinelli, et al., Contribution to the conference ICFDT_Frontier, in press at Nucl. Instr. Meth (2009).
Principal Publications and Authors
S. Almaviva, M. Marinelli, E. Milani, G. Prestopino, A. Tucciarone, C.Verona, G. Verona-Rinati, M. Angelone, M. Pillon,2 I. Dolbnya, K. Sawhney and N. Tartoni, J. Appl. Phys. 107, 014511 (2010).
Funding Acknowledgement
The diamond crystals were grown by the University of Rome “Tor Vergata” under a contract with Euratom.
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