??Technical Innovations and Notes

STABILIZATION OF RADIATION DAMAGE IN A SILICON DIODE RADIATION DETECTOR

DOUGLAS JONES, B.Sc. and DOUGLAS SCHUMACHER B.S.

Northwest Medical Physics Center, University of Washington. RC-08. Seattle, Washington 98 195

Measurements of the short circuit current generated by exposure in a Co-60 beam of a type IN2482 silicon diode have been made following irradiation by 6. 8. 12 and 18 MeV electrons. It is shown that the loss of sensitivity increases with both energy and dose. While the magnitude of the sensitivity loss depends on the individual diode. a dose on the order of 0.5 Mrad of 8 Me\’ electrons does serve to stabilize the sensitivity.

Radiotherapy. Quality control

INTRODUCTION An instrument for the rapid check of output, energy and field symmetry of supervoltage therapy equipment has been reported by the authors. This device originally was intended to monitor the X-ray beams from low energy linacs in use at radiotherapy centers in the Northwest. but has found application in monitoring electron beams from medium energy linacs. The change in sensitivity has been less than 5% per year when the device is used in X-ray beams in the energy range 4-6 MV, receiving on the order of 1.000 rad per day. However, when it was used for monitoring electron beams. a much more rapid decrease in sensitivity was noted. This experience is similar to that reported by other workers.’ The purpose of this study was to obtain information on the rate of sensitivity loss and investigate means to stabilize the diodes.

METHODS AND MATERIALS Eighteen type 112482 diodes of comparable sensitiv-

ity were selected from a batch of 477 diodes. The short circuit current in a Co-60 beam, which is referred to as the sensitivity, of the eighteen selected was close to the average sensitivity of the total group and thus the results obtained can be expected to be typical of this type of diode. The dose rate from a medium energy linac* at a depth of approximately 0.3 g - cm-’ in a plastic phantom. was established by routine techniques, for electron beams with initial energies 6. 12. and 18 MeV. The diodes were separated into three groups of six and exposed to accumu- lated doses of 5.000. 10.000 and 20.000 rad. The sensitiv- ity before and after each exposure was measured in a calibrated Co-60 beam all measurements normalized to

the sensitivity prior to the first cycle. The diodes then received a dose on the order of 0.5 Mrad using a 8MeV electron beam. Dose control during this process was not considered critical since measurements, which are to be described later in this paper. had demonstrated a satura- tion of the damage sites at this dose level. The exposure to 5.000, 10,000 and 20,000 from the electron beam was then repeated.

RESULTS Table 1 shows the sensitivity of the diodes normalized

to the sensitivity prior to the 5.000 rad dose, for both virgin and diodes hardened by the 0.5 Mrad dose. The dose required to harden the diodes was established by observing the sensitivity loss produced by a dose of 0.1 Mrad following doses up to 3 Mrad. Table 2 shows these results.

DISCUSSION The data presented in Table 2 demonstrates that a dose

of 0.5 Mrad minimizes subsequent radiation induced changes in the sensitivity of these diodes. It has been suggested that the damage mechanism is the displace- ment of silicon atoms.” Using the cross section data reported by 0en4 an increase in displacement density on the order of 508 would be expected in comparing 6 MeV and 18 MeV electrons. The data presented in Table 1 shows a four fold increase in damage when comparing 6 MeV and 18 MeV electrons. Up to 20.000 rad the damage appears to be proportional to dose and amounts to 1% at 6 MeV. 3.2% at 12 MeV. and 4.1% at 18 MeV for each 5.000 rad increment. It is interesting to note that the extent of damage appears to be diode dependent as demonstrated by the increase in the standard deviation of

??Varian. Clinac- 18 Reprint requests to: Douglas Jones. B.Sc.

Accepted for publication 4 October I979

110 Radiation Oncology 0 Biology ??Physics January 1980, Volume 6, No. 1

Table 1. Relative sensitivity after accumulated dose

Electron Diode energy number

5,000 rad 10.000 rad 20.000 rad Virgin Hardened Virgin Hardened Virgin Hardened

6 MeV

; 4 5 6

Average Standard Deviation

12 MeV 7 0.95 I 8 0.990 9 0.945

10 0.977 11 0.974 12 0.969

Average 0.969 Standard Deviation I .7%

18 MeV 13 14 15 I6 17 18

Average Standard Deviation

0.979 I.009 0.964 0.989 I .ooo 0.982 0.986 I .005 0.953 0.991 I .003 0.985 0.999 0.998 I .ooo 0.999 1.004 0.971 0.990 I .003 0.976 0.76% 0.4% I .7%

0.958 1.006 0.926 1.003 0.861 0.99 I 0.979 1.003 0.961 1.000 0.922 0.995 0.932 0.997 0.887 0.990 0.802 0.980 0.944 0.997 0.903 0.987 0.826 0.977 0.974 I .995 0.949 0.985 0.905 0.973 0.966 I .003 0.933 0.992 0.883 0.974 0.959 1.000 0.926 0.993 0.866 0.982 1.9% 0.4% 3.0% 0.7% 5.3% 0.9%

0.99 I 0.921 0.974 0.846 0.979 0.99 1 0.982 0.982 0.292 0.985 0.983 0.908 0.975 0.842 0.975 0.987 0.947 0.980 0.982 0.980 0.987 0.944 0.974 0.904 0.977 0.972 0.933 0.965 0.867 0.968 0.985 0.939 0.975 0.880 0.977 0.7% 2.7% 0.6% 3.9% 0.6%

1.018 0.933 I.007 0.967 I.010 0.940 I.003 0.970 I .OOo 0.997 I.016 0.997 1.009 0.959 0.7% 2.5%

1.027 I .007 I.015 I.006 I.000 1.020 1.013 I .O%

Table 2.

Sensitivity Loss Produced by Additional Total Dose Mrad 0. I Mrad Dose (%)

0.1 53 0.2 I8 0.4 8 0.6 3 0.9 3 I.1 6 3.1 4

I.

2.

the average sensitivity with increasing damage. Diodes subjected to a hardening process, which consists of irra- diation with 8 MeV electrons to a dose of 0.5 IMrad. exhibit minimal change in sensitivity in subsequent irra- diation. The possibility that diodes subjected to the hardening process recover their sensitivity was investi- gated by testing the sensitivity of a group of diodes at various times over a 42 day period following the harden- ing process. The sensitivity remained constant within the accuracy of the experiment which was estimated at +2%.

REFERENCES Deye, J.S., Padikal, T.N.: Experience with a pair of matched diodes for constancy checks on teletherapy equipment. Medical Physics 4: 352. 1977.

3.

Jones, D., Schumacher, D.: An instrument for the rapid check of output, energy and field symmetry of supervoltage therapy equipment. American Journal Roentgenol. 123: 198-202, 1975.

4.

McCall, R.C.. Jenkins, T.M., Oliver, G.D.: Photon response of silicon diode neutron detectors. Medical Physics 5: 37-4 I, 1978.

Oen. OS.: Cross section for atomic displacement in solids by fast electrons. ORNL-4897, Oak Ridge National Labora- tory, Oak Ridge. Tennessee 37830. 1973.