- Email: [email protected]
Development of direct temperature measurements of ISAC and ARIEL targets at TRIUMF
Nuclear Inst. and Methods in Physics Research B xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Development of direct temperature measurements of ISAC and ARIEL targets at TRIUMF A.S. Tanskanena,b, , A. Laxdalb, , P. Kunzb, M.R. Pearsonb, A. Shkuratoffb ⁎
a b
⁎
University of British Columbia, 2329 West Mall, Vancouver, British Columbia, Canada TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, Canada
ARTICLE INFO
ABSTRACT
Keywords: Radiometer Blackbody Emissivity ISAC ISOL TRIUMF
To improve the thermal modelling of high power isotope separation on-line (ISOL) targets at TRIUMF, an optical technique is being developed that allows for direct off-line and on-line temperature measurements of targets for radioactive isotope production. In this setup the light coming from a hot target through the ionizer opening is collected via a set of optics and coupled into a spectrometer. Thus, from the emission spectrum and Planck’s law, the target temperature can be deduced. Tantalum targets were heated to high temperatures and preliminary temperature measurements confirm the correlation between the spectrum of the radiation emitted from the target and the currents used to resistively heat the targets. The final goal is to apply this technique to on-line targets and correlate the isotope releases with the target temperatures for a better understanding of the diffusion and effusion processes happening in the target and for optimizing the delivery of short-lived species.
1. Introduction The ISAC facility at TRIUMF produces rare isotopes using the ISOL method [1]. A 500 MeV proton beam impinges onto the refractory target material, made of either metal foils or composite ceramic discs [2], inducing mainly spallation, fragmentation, and fission reactions. In order to enhance the release of the active products, the target is maintained at an elevated temperature using a combination of resistive and beam heating. The release of rare isotopes via diffusion and effusion is enhanced by a high target temperature. Each target material has a limiting operating temperature determined by the vapor pressure. If it is too high, the target can age prematurely and the efficiency of the attached ion source is reduced [3]. Therefore, an optical technique has been developed to measure the temperature of an operating ISOL target with a range up to 3000 K. 1.1. Motivation The aim of this project was to establish a method to measure and monitor the temperature of the on-line radioactive targets in the TRIUMF ARIEL/ISAC facility during beam delivery. To support the production of rare isotopes, a detailed knowledge of the absolute target
⁎
temperature and temperature uniformity is necessary. The acquisition of such knowledge combines both simulation and experimental data from off-line and on-line tests. Previous attempts in ISAC of using thermocouples on-line for temperature measurements of the target container had failed after a short time due to the high level of radiation. Pyrometers can be used but typically have insufficient spectral range. In addition, their optics and electronics are susceptible to degradation in a radioactive environment. The development of an optical radiometric temperature measurement technique specialized for ISOL targets would provide critical data to assist in benchmarking simulations and allow for further refinements of heating models. The studies include measurements of the extraction efficiency of various isotopes, which ultimately contribute to the optimization of target designs. A more precise knowledge of target temperature allows the mapping of the optimal release temperature of various isotopes. These studies first start with an experimental proof of principle test at the off-line test stand with stable beam, and then extend to the on-line target station, to be used during on-line beam delivery. This method also helps with monitoring the target degradation due to radiation aging and understanding the changes and differences in the temperature distribution with and without the proton beam.
Corresponding authors. E-mail addresses: [email protected] (A.S. Tanskanen), [email protected] (A. Laxdal).
https://doi.org/10.1016/j.nimb.2019.05.027 Received 31 January 2019; Received in revised form 11 April 2019; Accepted 9 May 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: A.S. Tanskanen, et al., Nuclear Inst. and Methods in Physics Research B, https://doi.org/10.1016/j.nimb.2019.05.027
Nuclear Inst. and Methods in Physics Research B xxx (xxxx) xxx–xxx
A.S. Tanskanen, et al.
2. Radiometer design
from 1035 nm to as high a value as allowable. The chosen optics and detector give a wavelength design bandwidth ranging from 930 nm to 1660 nm, allowing for blackbody emission peaks for temperatures ranging from 1750 K to 3100 K to be visualized.
2.1. Concept Optical emissions originate from the interior of the target cavity (Fig. 1) and are output through the ionizer opening. The emission spectra of the light emitted from the ionizer should closely follow Planck’s law assuming the target geometry is similar to that of a blackbody radiation source. Thus, if we can measure the emission spectra we can extract the cavity temperature from Planck’s law. For on-line measurements the device must be capable of acquiring a signal emitted through a 3 mm diameter ionizer opening at distances up to 8.7 m. Despite the kilowatts of power emitted by the hot targets, the large distances and intermediate apertures within the beamline mean that the signal will have relatively low power when it reaches the collection optics. Additionally, it is desirable to measure over as broad a wavelength bandwidth as possible ( 1000 nm) such that a wide range of colour temperature peaks can be visualized [4]. Thus, the collection optics must be achromatic in nature and well-matched with the numerical aperture of the optical fiber. Industry achromatic lenses were simulated in COMSOL Multiphysics and found to have a wide variance with wavelength in the amount of power coupled through a choice 200 μm multimode optical fiber over our design bandwidth. Considering these constraints and the high-radiation environment, front surface reflective optics are ideal as focusing elements. A parabolic mirror was used to focus light through a stacked linear optical fiber that passes the signal into a spectrometer (Fig. 2). The ISOL targets are known to reach temperatures up to 2500 K, which has an associated blackbody peak at roughly 1035 nm. Emission temperatures and blackbody peak wavelengths are inversely proportional, so it is ideal to measure wavelengths
2.2. Hardware and materials The resultant radiometer system is comprised of an Ocean Optics Flame NIR spectrometer with a Hammamatsu InGaAs photodetector with N = 128 spectral channels, a Thorlabs silver mirror collimator and an Ocean Optics 200 μm core linear stacked fiber with a built-in slit element. This results in a system with an operating range of 930–1660 nm. The optics are mounted on a four-axis adjustable mount (X, Y, pitch, and yaw) that is mounted on a portable camera tripod. 2.3. Calibration methods To calibrate the device it is necessary to gain spectral information at each discrete wavelength measured by the spectrometer. Performing a calibration with a pyrometer would provide spectral information for at most three discrete wavelengths and not account for the optical transmission of the system. In addition, operational experience at TRIUMF has shown thermocouples to be unreliable for absolute temperature measurements within the target. Thus, a Thorlabs stabilized tungsten–halogen lamp with a verified emission spectrum is used to calibrate the radiometer system. The spectra is recorded by the radiometer system over i = 1, 2, 3, …, 128 channels each corresponding to a discrete wavelength and will be of the form:
Ei ( i , T ) = D ( i) H ( i) Ss ( i , T )
(1)
where D ( i ) is the response curve of the InGaAs detector, H ( i ) is the combined transmission function for the optics and the glass apertures between the target and the detector, and Ss ( i , T ) is the emission spectra of the light source. For calibration, Ss ( i , T ) is the emission spectra of the tungsten-halogen lamp. Dividing the recorded signal of the spectrometer by the known spectra of the lamp results in the combined transmission function for the optics and the detector, G ( i ) = D ( i ) H ( i) . This transmission function can then be divided from each subsequent measurement giving
Vi =
Ei ( i , T ) G ( i)
Ss ( i , T )
(2)
where Vi is approximately equal to the true signal. 2.4. Temperature measurement After calibration, the detected signal at each of the spectral channels will be of the form
Fig. 1. ISAC low power tantalum target model.
Vi = Ai
i ( i,
T)
C1
5 exp (C2 / i T ), i
(3)
which is a simplified form of Planck’s Law according to Wien’s approximation, where: Ai is a proportionality constant of each channel, converting power density to detector counts, i ( i , T ) is the spectral emissivity, and C1 and C2 are the Planck’s law radiation constants. To extract the target temperature, a least squares curve fit of. Eq. (3) was performed on the calibrated data for both T and Ai . However, it must be noted that the emissivity for the measured signal is not exactly known, which highlights an inherent limitation with this method. The device will take N = 128 discrete measurements, but contain N + 1 unknowns due to the temperature and the unknown emissivity at each spectral channel. In the case of the ISAC targets, this can be remedied by considering that the cavity geometry closely resembles a blackbody [1,5]. Moreover, the spectral emissivity of refractory metals such as tantalum approaches a constant value at high temperatures and infrared wavelengths [6]. Thus, it is reasonable to assume that the
Fig. 2. Optical radiometer schematic measuring emissions from a hot ISOL target. At the (off-line) test stand L = 5.5 m and for the on-line target station L = 8.7 m. 2
Nuclear Inst. and Methods in Physics Research B xxx (xxxx) xxx–xxx
A.S. Tanskanen, et al.
line measurements. These measurements demonstrated a correlation between the spectra of the emissions and the heating current of the target (Fig. 3). Temperatures (Fig. 4) were extracted by fitting the calibrated readings to Eq. (3), where the constant emissivity was absorbed by the proportionality constant. 4. Future work Continued work on this project will involve implementing the radiometer on-line in ISAC for temperature measurements. A 20 m optical fiber will be used to isolate the sensitive components such as the spectrometer from the high radiation environment. The collection optics will be an optical path length of 8.7 m away from the emission source. Given that the signals obtained from 5.5 m away at the TRIUMF test stand required an integration time of 30 ms to prevent saturation and the on-line targets generate more intense emissions, we anticipate that this approach will remain feasible. Additionally, small deviations from a true blackbody radiation source have been observed. We plan to address these issues by either generating a mathematical model of the spectral emissivity of a hot target, or by extracting an emissivity curve as a function of wavelength and temperature from measurement.
Fig. 3. Curves showing the spectra of the emissions from an ISOL target cavity at the ISAC Test Stand. Curves are plotted for resistive heating values between I = 300 A and I = 600 A.
5. Summary and conclusions An optical radiometer has been designed and built for the study of target temperature to support the production of rare isotopes at TRIUMF. The system is optimized to allow for temperature measurement of ISOL targets radiating at colour temperatures up to 3000 K. Resulting from the target geometry, the emissions output from the ionizer aperture have behaviour close to that of a blackbody source, allowing for the approximation of a constant spectral emissivity. Acknowledgments This work was funded by TRIUMF which receives federal funding via a contribution agreement with the National Research Council of Canada. Fig. 4. Measured temperatures as a function of heating current applied to a test target in the ISAC test stand. The uncertainties shown are dominated by the systematic uncertainties in the optical transmission function and from deviations in the spectra from a true blackbody.
References [1] M. Dombsky, P. Bricault, High intensity targets for ISOL, historical and practical perspectives, Nucl. Instr. Meth. A 266 (1920) (2008) 4240424. [2] P. Kunz, P. Bricault, M. Dombsky, N. Erdmann, V. Hanemaayer, J. Wong, K. Lützenkirchen, Composite uranium carbide targets at TRIUMF: development and characterization with SEM, XRD, XRF and L-edge densitometry, J. Nuclear Mater. 440 (2013) 110. [3] Y. Zhang, G.D. Alton, Design of high-power ISOL targets for radioactive ion beam generation, Nuclear Instrum. Methods Phys. Res., Section A 521 (1) (2004) 72107. [4] P.A. Ni, F.M. Bieniosek, E. Henestroza, S.M. Lidia, A multiwavelength streak-opticalpyrometer for warm-dense matter experiments at NDCX-I and NDCX-II, Nuclear Instrum. Methods Phys. Res. A 733 (2014) 1217. [5] J.C. De Vos, Evaluation of the quality of a blackbody, Physica 20 (7-12) (1954) 669–689. [6] B.T. Barnes, Optical constants of incandescent refractory metals, J. Opt. Soc. Am. 56 (11) (1966) 1546–1550.
emissivity of the signals is that of a blackbody radiation source and constant over the design bandwidth for preliminary measurements. 3. Preliminary results Preliminary measurements were taken for a low power tantalum target at the ISAC test stand from a distance of 5.5 m (Fig. 2). At high heating currents the incident light saturated the spectrometer down to integration values of 25 ms, suggesting an adequate signal for future on-
3
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Development of direct temperature measurements of ISAC and ARIEL targets at TRIUMF A.S. Tanskanena,b, , A. Laxdalb, , P. Kunzb, M.R. Pearsonb, A. Shkuratoffb ⁎
a b
⁎
University of British Columbia, 2329 West Mall, Vancouver, British Columbia, Canada TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, Canada
ARTICLE INFO
ABSTRACT
Keywords: Radiometer Blackbody Emissivity ISAC ISOL TRIUMF
To improve the thermal modelling of high power isotope separation on-line (ISOL) targets at TRIUMF, an optical technique is being developed that allows for direct off-line and on-line temperature measurements of targets for radioactive isotope production. In this setup the light coming from a hot target through the ionizer opening is collected via a set of optics and coupled into a spectrometer. Thus, from the emission spectrum and Planck’s law, the target temperature can be deduced. Tantalum targets were heated to high temperatures and preliminary temperature measurements confirm the correlation between the spectrum of the radiation emitted from the target and the currents used to resistively heat the targets. The final goal is to apply this technique to on-line targets and correlate the isotope releases with the target temperatures for a better understanding of the diffusion and effusion processes happening in the target and for optimizing the delivery of short-lived species.
1. Introduction The ISAC facility at TRIUMF produces rare isotopes using the ISOL method [1]. A 500 MeV proton beam impinges onto the refractory target material, made of either metal foils or composite ceramic discs [2], inducing mainly spallation, fragmentation, and fission reactions. In order to enhance the release of the active products, the target is maintained at an elevated temperature using a combination of resistive and beam heating. The release of rare isotopes via diffusion and effusion is enhanced by a high target temperature. Each target material has a limiting operating temperature determined by the vapor pressure. If it is too high, the target can age prematurely and the efficiency of the attached ion source is reduced [3]. Therefore, an optical technique has been developed to measure the temperature of an operating ISOL target with a range up to 3000 K. 1.1. Motivation The aim of this project was to establish a method to measure and monitor the temperature of the on-line radioactive targets in the TRIUMF ARIEL/ISAC facility during beam delivery. To support the production of rare isotopes, a detailed knowledge of the absolute target
⁎
temperature and temperature uniformity is necessary. The acquisition of such knowledge combines both simulation and experimental data from off-line and on-line tests. Previous attempts in ISAC of using thermocouples on-line for temperature measurements of the target container had failed after a short time due to the high level of radiation. Pyrometers can be used but typically have insufficient spectral range. In addition, their optics and electronics are susceptible to degradation in a radioactive environment. The development of an optical radiometric temperature measurement technique specialized for ISOL targets would provide critical data to assist in benchmarking simulations and allow for further refinements of heating models. The studies include measurements of the extraction efficiency of various isotopes, which ultimately contribute to the optimization of target designs. A more precise knowledge of target temperature allows the mapping of the optimal release temperature of various isotopes. These studies first start with an experimental proof of principle test at the off-line test stand with stable beam, and then extend to the on-line target station, to be used during on-line beam delivery. This method also helps with monitoring the target degradation due to radiation aging and understanding the changes and differences in the temperature distribution with and without the proton beam.
Corresponding authors. E-mail addresses: [email protected] (A.S. Tanskanen), [email protected] (A. Laxdal).
https://doi.org/10.1016/j.nimb.2019.05.027 Received 31 January 2019; Received in revised form 11 April 2019; Accepted 9 May 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: A.S. Tanskanen, et al., Nuclear Inst. and Methods in Physics Research B, https://doi.org/10.1016/j.nimb.2019.05.027
Nuclear Inst. and Methods in Physics Research B xxx (xxxx) xxx–xxx
A.S. Tanskanen, et al.
2. Radiometer design
from 1035 nm to as high a value as allowable. The chosen optics and detector give a wavelength design bandwidth ranging from 930 nm to 1660 nm, allowing for blackbody emission peaks for temperatures ranging from 1750 K to 3100 K to be visualized.
2.1. Concept Optical emissions originate from the interior of the target cavity (Fig. 1) and are output through the ionizer opening. The emission spectra of the light emitted from the ionizer should closely follow Planck’s law assuming the target geometry is similar to that of a blackbody radiation source. Thus, if we can measure the emission spectra we can extract the cavity temperature from Planck’s law. For on-line measurements the device must be capable of acquiring a signal emitted through a 3 mm diameter ionizer opening at distances up to 8.7 m. Despite the kilowatts of power emitted by the hot targets, the large distances and intermediate apertures within the beamline mean that the signal will have relatively low power when it reaches the collection optics. Additionally, it is desirable to measure over as broad a wavelength bandwidth as possible ( 1000 nm) such that a wide range of colour temperature peaks can be visualized [4]. Thus, the collection optics must be achromatic in nature and well-matched with the numerical aperture of the optical fiber. Industry achromatic lenses were simulated in COMSOL Multiphysics and found to have a wide variance with wavelength in the amount of power coupled through a choice 200 μm multimode optical fiber over our design bandwidth. Considering these constraints and the high-radiation environment, front surface reflective optics are ideal as focusing elements. A parabolic mirror was used to focus light through a stacked linear optical fiber that passes the signal into a spectrometer (Fig. 2). The ISOL targets are known to reach temperatures up to 2500 K, which has an associated blackbody peak at roughly 1035 nm. Emission temperatures and blackbody peak wavelengths are inversely proportional, so it is ideal to measure wavelengths
2.2. Hardware and materials The resultant radiometer system is comprised of an Ocean Optics Flame NIR spectrometer with a Hammamatsu InGaAs photodetector with N = 128 spectral channels, a Thorlabs silver mirror collimator and an Ocean Optics 200 μm core linear stacked fiber with a built-in slit element. This results in a system with an operating range of 930–1660 nm. The optics are mounted on a four-axis adjustable mount (X, Y, pitch, and yaw) that is mounted on a portable camera tripod. 2.3. Calibration methods To calibrate the device it is necessary to gain spectral information at each discrete wavelength measured by the spectrometer. Performing a calibration with a pyrometer would provide spectral information for at most three discrete wavelengths and not account for the optical transmission of the system. In addition, operational experience at TRIUMF has shown thermocouples to be unreliable for absolute temperature measurements within the target. Thus, a Thorlabs stabilized tungsten–halogen lamp with a verified emission spectrum is used to calibrate the radiometer system. The spectra is recorded by the radiometer system over i = 1, 2, 3, …, 128 channels each corresponding to a discrete wavelength and will be of the form:
Ei ( i , T ) = D ( i) H ( i) Ss ( i , T )
(1)
where D ( i ) is the response curve of the InGaAs detector, H ( i ) is the combined transmission function for the optics and the glass apertures between the target and the detector, and Ss ( i , T ) is the emission spectra of the light source. For calibration, Ss ( i , T ) is the emission spectra of the tungsten-halogen lamp. Dividing the recorded signal of the spectrometer by the known spectra of the lamp results in the combined transmission function for the optics and the detector, G ( i ) = D ( i ) H ( i) . This transmission function can then be divided from each subsequent measurement giving
Vi =
Ei ( i , T ) G ( i)
Ss ( i , T )
(2)
where Vi is approximately equal to the true signal. 2.4. Temperature measurement After calibration, the detected signal at each of the spectral channels will be of the form
Fig. 1. ISAC low power tantalum target model.
Vi = Ai
i ( i,
T)
C1
5 exp (C2 / i T ), i
(3)
which is a simplified form of Planck’s Law according to Wien’s approximation, where: Ai is a proportionality constant of each channel, converting power density to detector counts, i ( i , T ) is the spectral emissivity, and C1 and C2 are the Planck’s law radiation constants. To extract the target temperature, a least squares curve fit of. Eq. (3) was performed on the calibrated data for both T and Ai . However, it must be noted that the emissivity for the measured signal is not exactly known, which highlights an inherent limitation with this method. The device will take N = 128 discrete measurements, but contain N + 1 unknowns due to the temperature and the unknown emissivity at each spectral channel. In the case of the ISAC targets, this can be remedied by considering that the cavity geometry closely resembles a blackbody [1,5]. Moreover, the spectral emissivity of refractory metals such as tantalum approaches a constant value at high temperatures and infrared wavelengths [6]. Thus, it is reasonable to assume that the
Fig. 2. Optical radiometer schematic measuring emissions from a hot ISOL target. At the (off-line) test stand L = 5.5 m and for the on-line target station L = 8.7 m. 2
Nuclear Inst. and Methods in Physics Research B xxx (xxxx) xxx–xxx
A.S. Tanskanen, et al.
line measurements. These measurements demonstrated a correlation between the spectra of the emissions and the heating current of the target (Fig. 3). Temperatures (Fig. 4) were extracted by fitting the calibrated readings to Eq. (3), where the constant emissivity was absorbed by the proportionality constant. 4. Future work Continued work on this project will involve implementing the radiometer on-line in ISAC for temperature measurements. A 20 m optical fiber will be used to isolate the sensitive components such as the spectrometer from the high radiation environment. The collection optics will be an optical path length of 8.7 m away from the emission source. Given that the signals obtained from 5.5 m away at the TRIUMF test stand required an integration time of 30 ms to prevent saturation and the on-line targets generate more intense emissions, we anticipate that this approach will remain feasible. Additionally, small deviations from a true blackbody radiation source have been observed. We plan to address these issues by either generating a mathematical model of the spectral emissivity of a hot target, or by extracting an emissivity curve as a function of wavelength and temperature from measurement.
Fig. 3. Curves showing the spectra of the emissions from an ISOL target cavity at the ISAC Test Stand. Curves are plotted for resistive heating values between I = 300 A and I = 600 A.
5. Summary and conclusions An optical radiometer has been designed and built for the study of target temperature to support the production of rare isotopes at TRIUMF. The system is optimized to allow for temperature measurement of ISOL targets radiating at colour temperatures up to 3000 K. Resulting from the target geometry, the emissions output from the ionizer aperture have behaviour close to that of a blackbody source, allowing for the approximation of a constant spectral emissivity. Acknowledgments This work was funded by TRIUMF which receives federal funding via a contribution agreement with the National Research Council of Canada. Fig. 4. Measured temperatures as a function of heating current applied to a test target in the ISAC test stand. The uncertainties shown are dominated by the systematic uncertainties in the optical transmission function and from deviations in the spectra from a true blackbody.
References [1] M. Dombsky, P. Bricault, High intensity targets for ISOL, historical and practical perspectives, Nucl. Instr. Meth. A 266 (1920) (2008) 4240424. [2] P. Kunz, P. Bricault, M. Dombsky, N. Erdmann, V. Hanemaayer, J. Wong, K. Lützenkirchen, Composite uranium carbide targets at TRIUMF: development and characterization with SEM, XRD, XRF and L-edge densitometry, J. Nuclear Mater. 440 (2013) 110. [3] Y. Zhang, G.D. Alton, Design of high-power ISOL targets for radioactive ion beam generation, Nuclear Instrum. Methods Phys. Res., Section A 521 (1) (2004) 72107. [4] P.A. Ni, F.M. Bieniosek, E. Henestroza, S.M. Lidia, A multiwavelength streak-opticalpyrometer for warm-dense matter experiments at NDCX-I and NDCX-II, Nuclear Instrum. Methods Phys. Res. A 733 (2014) 1217. [5] J.C. De Vos, Evaluation of the quality of a blackbody, Physica 20 (7-12) (1954) 669–689. [6] B.T. Barnes, Optical constants of incandescent refractory metals, J. Opt. Soc. Am. 56 (11) (1966) 1546–1550.
emissivity of the signals is that of a blackbody radiation source and constant over the design bandwidth for preliminary measurements. 3. Preliminary results Preliminary measurements were taken for a low power tantalum target at the ISAC test stand from a distance of 5.5 m (Fig. 2). At high heating currents the incident light saturated the spectrometer down to integration values of 25 ms, suggesting an adequate signal for future on-
3