- Email: [email protected]
Flux monitor diode radiation hardness testing
Nuclear Instruments and Methods in Physics Research A 652 (2011) 112–115
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Flux monitor diode radiation hardness testing M.L. Lombardi a,b,n, A. Favalli a, J.M. Goda a, K.D. Ianakiev a, C.E. Moss a a b
Nuclear Nonproliferation Division, Los Alamos National Laboratory, PO Box 1663, Los Alamos, NM 87545, USA University of New Mexico, Albuquerque, NM 87131, USA
a r t i c l e in f o
a b s t r a c t
Available online 30 August 2010
A flux monitor diode is being explored as an option for measurement of the output of an X-ray tube that is used for active transmission measurements on a pipe containing UF6 gas. The measured flux can be used to correct for any instabilities in the X-ray tube or the high voltage power supply. For this measurement, we are using a silicon junction p–n photodiode, model AXUV100GX, developed by International Radiation Detectors, Inc. (IRD, Inc.). This diode has a silicon thickness of 104 m and a thin (3–7 nm) silicon dioxide junction passivating, protective entrance window. These diodes have been extensively tested for radiation hardness in the UV range. However, we intend to operate mainly in the 10–40 keV X-ray region. We are performing radiation hardness testing over this energy range, with the energy spectrum that would pass through the diode during normal operation. A long-term measurement was performed at a high flux, which simulated over 80 years of operation. No significant degradation was seen over this time. Fluctuations were found to be within the 0.1% operationally acceptable error range. After irradiation, an I–V characterization showed a temporary irradiation effect which decayed over time. This effect is small because we operate the diode without external bias. Published by Elsevier B.V.
Keywords: Diode radiation hardness X-ray tube Enrichment monitor Gas centrifuge enrichment plant
1. Introduction
2. Experimental setup
We have been working to develop a modern enrichment monitoring technology for online monitoring of UF6 enrichment in the header pipes of Gaseous Centrifuge Enrichment Plants (GCEPs) [1]. This technology is based on a transmission measurement of the UF6 gas density using an X-ray tube and a transmission peak generated with a notch filter. We use the K-edge absorption in the thin filter material, such as silver, to convert the bremsstrahlung distribution from the X-ray tube into a peak at the K-edge energy. The choice of filter material gives us flexible selection of the transmission peak energy. For example, silver has a K-edge energy of 25.5 keV. Using an X-ray tube will allow us to avoid the high replacement and recalibration cost caused by a decaying radioisotope source, such as 109Cd, which is generally used in facilities with aluminum pipes. The main issue with the X-ray source is instability of the high voltage. A silicon flux monitor diode placed in the X-ray beam allows us to correct for instabilities. This detector operates at zero voltage and has relatively low temperature sensitivity. The setup is shown in Fig. 1, with an inset of the diode in the upper right.
The following analytical formula is used to describe the energy spectrum seen by the flux monitor diode:
n Corresponding author at: Los Alamos National Laboratory, MS B228 PO Box 1663, Los Alamos, NM 87545, USA. Tel.: + 1 505 667 4116; fax: + 1 505 665 9849. E-mail address: [email protected] (M.L. Lombardi).
0168-9002/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.nima.2010.08.066
n h i Ec I¼k 1 exp mAg ðEÞrAg dAg E
ð1Þ
The intensity of the energy spectrum, I, is the bremsstrahlung yield of the tube [2] multiplied by the attenuation in the silver notch filter. The terms mAg, rAg, and dAg are the mass attenuation coefficient, density, and thickness of the silver notch filter, respectively, and n is an constant that depends on the anode material of the X-ray tube. We used a tube with a palladium anode, for which n¼ 0.97. This value was taken from interpolation of the data in [2]. We set Ec, the cutoff voltage of the tube, to 40 kV. This is a typical setting that would be used in operation in a GCEP. We are then able to increase the beam current to increase the flux at the diode, in order to simulate very long operating times. Fig. 2 shows the calculated spectrum that is generated by the Ag attenuated X-ray tube beam. Although the 0.1 mm-thick silver notch filter used for the hardness testing is thinner than what would be used in normal operation by a factor of between two and five, it still gives us the typical transmission spectrum to which the diode would be subjected.
M.L. Lombardi et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 112–115
113
power. The International Radiation Detectors, Inc. (IRD, Inc.; website www.ird-inc.com) compares experimental responsivity results for a calibrated diode to this theoretical formula, which shows excellent agreement [3]. We use this responsivity curve (Fig. 3) to determine the photon flux at the detector. The following equation relates the responsivity to the photon flux: R¼
Fig. 1. Experimental setup, showing X-ray tube and an inset of the diode. The X-rays are directed into steel shielding with the diode at the far end. The shielded cable connects to the Keithley picoammeter for the readout of the diode current.
Iex
fE
ð2Þ
where R is the responsivity in amperes per watt, Iex is the average measured current in the diode during irradiation, f is the flux in photons/s, and E is the approximate irradiation energy in keV. This equation is valid assuming a monoenergetic photon beam impinging the detector surface. If we use an average energy of 22.3 keV for E (the actual spectrum is discussed in Section 2), then Fig. 2 gives us the diode responsivity. Because we are producing a narrow energy distribution with the notch filter, this approximation is applicable. Iex was measured in our experiment. Substituting these values and solving for f, we calculated a flux of 1.4 1010 photons/s. We used this flux to calculate a dose rate and total dose. We determined the absorbed dose in the diode during the long term (30 days) irradiation to be about 40 rad or 55 mrad/h. The dose and dose rate calculation were performed using the MCNPX code [4], using an F6 (energy deposition) tally with a beam of photons directed at the silicon diode. An energy distribution was created to approximate the spectrum shown in Fig. 2.
4. Long-term irradiation measurement
Fig. 2. Spectrum generated by X-ray tube (Pd anode) with 0.1 mm-thick silver notch filter.
We irradiated the diode at a tube voltage of 40 kV and a beam current of 1 mA. With the 0.1 mm-thick silver filter in place, an average current of 3491 nA was induced in the diode. The average diode current was measured with a Keithley picoammeter Model 6485. A readout from the board of the X-ray tube high voltage power supply provided the temperature data. Fig. 4 shows the temperature-corrected diode current over the 30-day period of irradiation. A temperature correction factor (change in diode output current per degree centigrade) had been obtained previously for the diode being characterized. This factor was obtained placing the diode in an environmental chamber and using a temperature profile of 25–5 1C and 45–25 1C. Each temperature was maintained for at least 1 h to allow the diode to reach equilibrium, and a slow ramp rate of between four and five degrees per hour was used. High voltage also is plotted to demonstrate that most of the major fluctuations in the temperature-corrected data can be attributed to changes in the high voltage, which is also affected by temperature. The narrow drops in the current are unexplained instrumental artifacts; however, even with these, the temperature-corrected current falls within the acceptable 0.1% error range. This operation of the diode at a flux of 1000 times that of normal operation for 30 days is roughly equivalent to 80 years of normal operation.
Fig. 3. Diode responsivity as a function of energy. Data taken from the IRD website [3].
5. Results and discussion 3. Diode responsivity We use the theoretical responsivity of the flux monitor diode as a function of X-ray energy, for the 104 m silicon thickness, to determine the absorbed dose in the diode over the long-term irradiation. Responsivity is defined as a measure of the amount of output current produced by the diode for an incident radiant
An I–V characterization of the diode was performed following the long-term irradiation. Fig. 5 shows the measured curves, over a 13-day period. We held the diode at 25 1C for the first 7 days after irradiation and recorded the decay. At this point the diode was heated to 45 1C. The final two measurements shown in Fig. 5 are taken with the diode back at 25 1C. Additional measurements showed no significant change with time.
114
M.L. Lombardi et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 112–115
Fig. 4. Long-term test at 40 kV and 1 mA. Temperature-corrected diode data are shown, plus high voltage to help explain the remaining fluctuations.
Fig. 5. I–V characterization after irradiation.
sensitivity to this effect because we operate the diodes without external bias (very close to 0 V). No permanent damage to the diode appears to have been done by any of the long-term measurements. Small fluctuations of the measured current were seen, but these can be explained by temperature changes in the room and fluctuations in the X-ray tube’s high voltage.
Fig. 6. Observed recovery of the diodes after irradiation.
Prolonged ionizing radiation can cause charge trapping (ionizing damage) in the oxide layer of the diode and at the oxide/silicon interface [5]. After the diode was irradiated, we observed first a fast (short-term) recovery followed by a slow (long-term) recovery. This recovery is shown in Fig. 6, for three different voltages on the I–V curve. Increasing the temperature it is possible to free the charge stored in the oxide. We expect little
6. Conclusion We see no permanent damage done to the silicon diode with X-ray energies that are useful for performing transmission-based enrichment measurements in a GCEP. The diode received an absorbed dose of about 40 rad, with no observed degradation in its operation. Therefore, these flux monitors will be an important tool for stabilizing X-ray tubes during the long durations of possibly unattended monitoring in a facility. It appears that a temporary charge-trapping effect may exist that was caused by irradiation of the diodes with X-rays in the 10–40 keV region.
M.L. Lombardi et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 112–115
Future work will include the characterization of additional diodes from IRD, Inc. and the testing of diodes from other manufacturers.
Acknowledgements The authors would like to thank Craig McCluskey for performing the temperature characterization of the diode. This work was supported by the United States Department of Energy National Nuclear Security Administration Office for Nonproliferation Research and Development (NA-22) and the Office of Nuclear Noncompliance Verification (NA-241).
115
References [1] K.D. Ianakiev, et al., Progress in development of an advanced enrichment monitor based on transmission measurements with an X-Ray source and NaI(Tl) spectrometer, in: Proceedings of the 50th Annual INMM Meeting, Tucson, AZ, July 12–16, 2009. [2] G.H. McCall, J. Phys. D: Appl. Phys. 15 (1982) 823. [3] International Radiation Detectors, Inc., Absolute X-ray detectors. /www. ird-inc.comS (accessed June 15, 2010). [4] /https://mcnpx.lanl.govS (accessed June 15, 2010). [5] A.H. Siedle, L. Adams, Handbook of Radiation Effects, second ed., Oxford University Press, 2002.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Flux monitor diode radiation hardness testing M.L. Lombardi a,b,n, A. Favalli a, J.M. Goda a, K.D. Ianakiev a, C.E. Moss a a b
Nuclear Nonproliferation Division, Los Alamos National Laboratory, PO Box 1663, Los Alamos, NM 87545, USA University of New Mexico, Albuquerque, NM 87131, USA
a r t i c l e in f o
a b s t r a c t
Available online 30 August 2010
A flux monitor diode is being explored as an option for measurement of the output of an X-ray tube that is used for active transmission measurements on a pipe containing UF6 gas. The measured flux can be used to correct for any instabilities in the X-ray tube or the high voltage power supply. For this measurement, we are using a silicon junction p–n photodiode, model AXUV100GX, developed by International Radiation Detectors, Inc. (IRD, Inc.). This diode has a silicon thickness of 104 m and a thin (3–7 nm) silicon dioxide junction passivating, protective entrance window. These diodes have been extensively tested for radiation hardness in the UV range. However, we intend to operate mainly in the 10–40 keV X-ray region. We are performing radiation hardness testing over this energy range, with the energy spectrum that would pass through the diode during normal operation. A long-term measurement was performed at a high flux, which simulated over 80 years of operation. No significant degradation was seen over this time. Fluctuations were found to be within the 0.1% operationally acceptable error range. After irradiation, an I–V characterization showed a temporary irradiation effect which decayed over time. This effect is small because we operate the diode without external bias. Published by Elsevier B.V.
Keywords: Diode radiation hardness X-ray tube Enrichment monitor Gas centrifuge enrichment plant
1. Introduction
2. Experimental setup
We have been working to develop a modern enrichment monitoring technology for online monitoring of UF6 enrichment in the header pipes of Gaseous Centrifuge Enrichment Plants (GCEPs) [1]. This technology is based on a transmission measurement of the UF6 gas density using an X-ray tube and a transmission peak generated with a notch filter. We use the K-edge absorption in the thin filter material, such as silver, to convert the bremsstrahlung distribution from the X-ray tube into a peak at the K-edge energy. The choice of filter material gives us flexible selection of the transmission peak energy. For example, silver has a K-edge energy of 25.5 keV. Using an X-ray tube will allow us to avoid the high replacement and recalibration cost caused by a decaying radioisotope source, such as 109Cd, which is generally used in facilities with aluminum pipes. The main issue with the X-ray source is instability of the high voltage. A silicon flux monitor diode placed in the X-ray beam allows us to correct for instabilities. This detector operates at zero voltage and has relatively low temperature sensitivity. The setup is shown in Fig. 1, with an inset of the diode in the upper right.
The following analytical formula is used to describe the energy spectrum seen by the flux monitor diode:
n Corresponding author at: Los Alamos National Laboratory, MS B228 PO Box 1663, Los Alamos, NM 87545, USA. Tel.: + 1 505 667 4116; fax: + 1 505 665 9849. E-mail address: [email protected] (M.L. Lombardi).
0168-9002/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.nima.2010.08.066
n h i Ec I¼k 1 exp mAg ðEÞrAg dAg E
ð1Þ
The intensity of the energy spectrum, I, is the bremsstrahlung yield of the tube [2] multiplied by the attenuation in the silver notch filter. The terms mAg, rAg, and dAg are the mass attenuation coefficient, density, and thickness of the silver notch filter, respectively, and n is an constant that depends on the anode material of the X-ray tube. We used a tube with a palladium anode, for which n¼ 0.97. This value was taken from interpolation of the data in [2]. We set Ec, the cutoff voltage of the tube, to 40 kV. This is a typical setting that would be used in operation in a GCEP. We are then able to increase the beam current to increase the flux at the diode, in order to simulate very long operating times. Fig. 2 shows the calculated spectrum that is generated by the Ag attenuated X-ray tube beam. Although the 0.1 mm-thick silver notch filter used for the hardness testing is thinner than what would be used in normal operation by a factor of between two and five, it still gives us the typical transmission spectrum to which the diode would be subjected.
M.L. Lombardi et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 112–115
113
power. The International Radiation Detectors, Inc. (IRD, Inc.; website www.ird-inc.com) compares experimental responsivity results for a calibrated diode to this theoretical formula, which shows excellent agreement [3]. We use this responsivity curve (Fig. 3) to determine the photon flux at the detector. The following equation relates the responsivity to the photon flux: R¼
Fig. 1. Experimental setup, showing X-ray tube and an inset of the diode. The X-rays are directed into steel shielding with the diode at the far end. The shielded cable connects to the Keithley picoammeter for the readout of the diode current.
Iex
fE
ð2Þ
where R is the responsivity in amperes per watt, Iex is the average measured current in the diode during irradiation, f is the flux in photons/s, and E is the approximate irradiation energy in keV. This equation is valid assuming a monoenergetic photon beam impinging the detector surface. If we use an average energy of 22.3 keV for E (the actual spectrum is discussed in Section 2), then Fig. 2 gives us the diode responsivity. Because we are producing a narrow energy distribution with the notch filter, this approximation is applicable. Iex was measured in our experiment. Substituting these values and solving for f, we calculated a flux of 1.4 1010 photons/s. We used this flux to calculate a dose rate and total dose. We determined the absorbed dose in the diode during the long term (30 days) irradiation to be about 40 rad or 55 mrad/h. The dose and dose rate calculation were performed using the MCNPX code [4], using an F6 (energy deposition) tally with a beam of photons directed at the silicon diode. An energy distribution was created to approximate the spectrum shown in Fig. 2.
4. Long-term irradiation measurement
Fig. 2. Spectrum generated by X-ray tube (Pd anode) with 0.1 mm-thick silver notch filter.
We irradiated the diode at a tube voltage of 40 kV and a beam current of 1 mA. With the 0.1 mm-thick silver filter in place, an average current of 3491 nA was induced in the diode. The average diode current was measured with a Keithley picoammeter Model 6485. A readout from the board of the X-ray tube high voltage power supply provided the temperature data. Fig. 4 shows the temperature-corrected diode current over the 30-day period of irradiation. A temperature correction factor (change in diode output current per degree centigrade) had been obtained previously for the diode being characterized. This factor was obtained placing the diode in an environmental chamber and using a temperature profile of 25–5 1C and 45–25 1C. Each temperature was maintained for at least 1 h to allow the diode to reach equilibrium, and a slow ramp rate of between four and five degrees per hour was used. High voltage also is plotted to demonstrate that most of the major fluctuations in the temperature-corrected data can be attributed to changes in the high voltage, which is also affected by temperature. The narrow drops in the current are unexplained instrumental artifacts; however, even with these, the temperature-corrected current falls within the acceptable 0.1% error range. This operation of the diode at a flux of 1000 times that of normal operation for 30 days is roughly equivalent to 80 years of normal operation.
Fig. 3. Diode responsivity as a function of energy. Data taken from the IRD website [3].
5. Results and discussion 3. Diode responsivity We use the theoretical responsivity of the flux monitor diode as a function of X-ray energy, for the 104 m silicon thickness, to determine the absorbed dose in the diode over the long-term irradiation. Responsivity is defined as a measure of the amount of output current produced by the diode for an incident radiant
An I–V characterization of the diode was performed following the long-term irradiation. Fig. 5 shows the measured curves, over a 13-day period. We held the diode at 25 1C for the first 7 days after irradiation and recorded the decay. At this point the diode was heated to 45 1C. The final two measurements shown in Fig. 5 are taken with the diode back at 25 1C. Additional measurements showed no significant change with time.
114
M.L. Lombardi et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 112–115
Fig. 4. Long-term test at 40 kV and 1 mA. Temperature-corrected diode data are shown, plus high voltage to help explain the remaining fluctuations.
Fig. 5. I–V characterization after irradiation.
sensitivity to this effect because we operate the diodes without external bias (very close to 0 V). No permanent damage to the diode appears to have been done by any of the long-term measurements. Small fluctuations of the measured current were seen, but these can be explained by temperature changes in the room and fluctuations in the X-ray tube’s high voltage.
Fig. 6. Observed recovery of the diodes after irradiation.
Prolonged ionizing radiation can cause charge trapping (ionizing damage) in the oxide layer of the diode and at the oxide/silicon interface [5]. After the diode was irradiated, we observed first a fast (short-term) recovery followed by a slow (long-term) recovery. This recovery is shown in Fig. 6, for three different voltages on the I–V curve. Increasing the temperature it is possible to free the charge stored in the oxide. We expect little
6. Conclusion We see no permanent damage done to the silicon diode with X-ray energies that are useful for performing transmission-based enrichment measurements in a GCEP. The diode received an absorbed dose of about 40 rad, with no observed degradation in its operation. Therefore, these flux monitors will be an important tool for stabilizing X-ray tubes during the long durations of possibly unattended monitoring in a facility. It appears that a temporary charge-trapping effect may exist that was caused by irradiation of the diodes with X-rays in the 10–40 keV region.
M.L. Lombardi et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 112–115
Future work will include the characterization of additional diodes from IRD, Inc. and the testing of diodes from other manufacturers.
Acknowledgements The authors would like to thank Craig McCluskey for performing the temperature characterization of the diode. This work was supported by the United States Department of Energy National Nuclear Security Administration Office for Nonproliferation Research and Development (NA-22) and the Office of Nuclear Noncompliance Verification (NA-241).
115
References [1] K.D. Ianakiev, et al., Progress in development of an advanced enrichment monitor based on transmission measurements with an X-Ray source and NaI(Tl) spectrometer, in: Proceedings of the 50th Annual INMM Meeting, Tucson, AZ, July 12–16, 2009. [2] G.H. McCall, J. Phys. D: Appl. Phys. 15 (1982) 823. [3] International Radiation Detectors, Inc., Absolute X-ray detectors. /www. ird-inc.comS (accessed June 15, 2010). [4] /https://mcnpx.lanl.govS (accessed June 15, 2010). [5] A.H. Siedle, L. Adams, Handbook of Radiation Effects, second ed., Oxford University Press, 2002.