Stable Semiconductor Magnetic Field Sensors Under Dozes of High

30th EPS Conference on Contr. Fusion and Plasma Phys., St. Petersburg, 7-11 July 2003 ECA Vol. 27A, P-4.68

Stable Semiconductor Magnetic Field Sensors Under Dozes of High Radiation. I. Bolshakova1, V. Brudnyi2, R. Holyaka1, N. Kolin3, M. Kumada4, C. Leroy5 1

Magnetic Sensor Laboratory, Lviv Polytechnic National University, Lviv, Ukraine 2 Siberian Physical Technical Institute, Tomsk, Russia 3 Obninsk Branch State Research Center "Karpov Institute of Physical Chemistry", Obninsk, Russia 4 National Institute of Radiological Sciences, Chiba, Japan 5 Laboratory R-J. A.-Levesque, Montreal University, Montreal, Canada Abstract The influence of irradiation by fast neutrons on characteristics of InSb semiconductor mi-

crocrystals that are used for creation of Hall radiation resistant magnetic field microsensors has been investigated. It is reported about an experiment on measuring radiation resistant microsensors directly under the irradiation at the IBR-2 reactor channel and about their application in magnetic measurement systems. It is shown that radiation resistance of microsensors is not worse than 0.04% in case of their irradiation by fast neutrons up to a fluence of 1⋅1015 n⋅cm-2. Introduction. Characteristics of ordinary semiconductor materials and devices on their base change greatly under the influence of radiation. For example, silicon transistors are failing at a fast neutron fluence of 10 15 n⋅cm-2, and GaAs light-emitting diodes and photo diodes at a fast neutron fluence of 1014 n⋅cm-2. Sensitivity to magnetic field is one of the basic characteristics, and in the well known Hall sensors it changes for several percents under the influence of irradiation by fast neutrons up to a fluence of 1015 n⋅cm-2. This makes precise magnetic field measurements using this kind of sensors impossible. All the more so it is impossible to perform magnetic field monitoring in radiation conditions within a long period of time. Creation of magnetic measurement aids, stable under dozes of high radiation will allow to increase the accuracy of the magnetic field measurement in conditions of charged particle accelerators and tokamaks. In the paper it is reported about the creation of radiation resistant semiconductor materials and magnetic field sensors, about the results of their tests in the high-energy neutrons flux and about their possible application. Semiconductor materials for radiation resistant sensors. At the Magnetic Sensor Laboratory LPNU the technology of obtaining III-V semiconductor compounds stable under dozes of high radiation has been developed. At the present time the most stable in conditions of irradiation by fast neutrons are the InSb microcrystals doped with optimal impurity complex during the growth process. The basic element in this complex is


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Sn. Selection of Sn as a primary doping impurity has been made considering that as a result of indium radiation transformations in case of its interaction with thermal neutrons, that are partially present in a fast neutron flux, the end product of a nuclear reactions is stannum. The other doping components of impurity complex must interact with residual impurities and transfer them into inactive state and also create drains for radiation defects. In the Fig. 1 it is shown that relative charge carrier concentration change depends on initial carrier concentration level in doped InSb microcrystals, irradiated by a fast neutron fluence of F=1.0⋅1015 n⋅cm-2, it tends to minimum while doping level increases and it does not exceed 0.04% for initial charge carrier concentration of n=6.4⋅1017 cm-3, that is optimal.

Fig. 1.Influence of irradiation by fast neutrons on charge carrier concentration (n) in sensor microcrystals The results shown in the Fig. 1 are obtained as follows: characteristics of InSb microcrystal samples doped with impurity complexes of different composition have been measured at precision benches, created at the Laboratory on the base of HMS-7504 facility (LakeShore, USA). Then they have been sent to the Neutron Physics Laboratory, JINR, Dubna and exposed to fast neutrons at the IBR-2 pulsed reactor. After the residual radiation had disappeared, samples have been returned to the laboratory and their characteristics have been measured at the same benches with accurate respect to the original conditions. The accuracy of determining relative charge carrier concentration change is equal to 0.01%. Investigation of radiation resistant magnetic field sensors during the irradiation. Hall microsensors created on the base of InSb microcrystals have dimensions of (0.05x0.05x0.02)mm3 and magnetic field sensitivity 50 mV/T at operation current of 10 mA. Operation temperature range is 4.2÷300 K, measurable magnetic field range is 0.1÷10 T. Because time interval between samples irradiation and measurement of their characteristics after irradiation in the previous experiments was quite long (from several days to a several weeks), the results obtained may include after-effects of relaxation processes, which


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could be possible in materials with high carrier mobility even at quite low temperatures. So an experiment on investigation of radiation resistant sensors directly under the irradiation by fast neutrons have been prepared and carried out at IBR-2 reactor. Microsensor samples have been placed in a gap between the poles of SmCo5 permanent magnet and together they were put into the reactor channel. Measurement of sensor output signals have been carried out using magnetic measurement system specially created at the Laboratory, distanced by 30m from reactor channel. Sensor signals as well as samples supply current have been transmitted via special cable which noise immunity had been provided by the measurement system. Besides, measurement system has function of samples temperature measurement with accuracy up to 0.1°C and control of permanent magnet magnetic induction change under the radiation influence. System measurement channels error is 0.01%.

Fig. 2. Change in sensor sensitivity measured during irradiation by fast neutrons The experiment has lasted for three month and it has finished on February 28th, 2003. The operation schedule of the reactor during that period is shown at the Fig. 2. Fast neutron energy is

f

= 0.1÷13 MeV, flux density is ϕf = 6.9⋅109 n⋅cm-2s-1÷1.7⋅1010 n⋅cm-2s-1 for dif-

ferent sessions. In the Fig. 2 there are given test results of radiation hardness for three microsensors with different doping levels and initial carrier concentrations being investigated. During three sessions microsensor samples were irradiated up to a fluence of 3.1⋅1016 n⋅"m-2. As one may see from the figure, for samples with optimal initial carrier concentration of n=6.4⋅1017 "m-3 sensitivity change does not exceed 1% at the irradiation up to a fluence of F=3.1⋅1016 n⋅"m-2. At the same time sensitivity of ordinary samples has changed under these conditions considerably and value of the change is > 24%. From the curves in the Fig. 2 one may see that signals of all sensors did not change during the reactor stop. The behavior of investigated samples up to a fluence of F=1.0⋅1015 n⋅"m-2 is of a particular interest. This level is standard of Russia for material radiation hardness evaluation.


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And besides, such fast neutron fluence F=1015 n⋅"m-2 is achieved at detector complexes ATLAS and LMS colliders of LHC. Fluence of F=1⋅1015 n⋅"m-2 was achieved within first 41 hours of irradiation. Results of samples investigation are given in Fig. 3. From this figure follows that sensitivity change of sensors with optimal initial carrier concentration n=6.4⋅1017 cm-3 does not exceed 0.04%.

Fig. 3. Change in sensor sensitivity measured during irradiation by fast neutrons Radiation resistant magnetic field sensors are used in the magnetic measurement system developed at the Laboratory for magnetic field monitoring in charged particle accelerators. The system is intelligent, because it has self diagnostic and self correction functions [2]. Sensors that are stable in radiation conditions are applied also in “Microsuputnyk” and “EgyptSat-1” magnetic systems of spacecraft control that are under design at SDO “Pivdenne” (Dnepropetrovsk, Ukraine). Magnetic field mappers have been developed and manufactured at the laboratory on the basis of for investigation of medical purpose cyclotron magnets developed at National Institute of Radiological Sciences, Japan. Conclusions. Magnetic field microsensors stable under dozes of high radiation on the base of radiation resistant semiconductor microcrystals are created. Sensitivity of this sensors change for no more than 0.04% under the irradiation by fast neutrons up to a fluence of 1015 n⋅cm-2. Such radiation resistant sensors are used in magnetic measurement system for magnetic field monitoring in charged particle accelerators, in the open Space and in measurement systems for cyclotron magnet mapping. References 1. V.Ivleva, M.Kevorkov et al.// Sov. Phys. Semiconductors, 2001, v.61 – p.743-745. 2. Bolshakova I., Holyaka R., Leroy C. Novel approaches towards the development of Hall sensor-based magnetometric devices for charged particle accelerators // IEEE Transactions on Applied Superconductivity. – 2002. – V. 12. – $1. - P. 1655-1658.