- Email: [email protected]
Extending the measurement temperature range in a fully automated luminescence reader to −50 °C
Radiation Measurements 124 (2019) 13–18
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Extending the measurement temperature range in a fully automated luminescence reader to −50 °C
T
Michael Dischera,∗, Kay Dornichb, Andreas Richterb, Barbara Mauza, Andreas Langa a b
Department of Geography and Geology, University of Salzburg, Hellbrunner Str. 34, 5020, Salzburg, Austria Freiberg Instruments GmbH, Delfter Str. 6, 09599, Freiberg, Germany
ARTICLE INFO
ABSTRACT
Keywords: Luminescence equipment development Cooling unit Novel instrumentation
It is today well established that the luminescence signal is temperature-dependent with compelling evidence provided for temperatures above room temperature. For the signal temperature dependence on lower temperatures, however, our knowledge is rather limited due to the lack of luminescence readers that allow cooling as well as heating in an automated setup. Here, we present technical details and test results of an upgraded Lexsyg Research luminescence reader equipped with a thermoelectric cooling device: a Peltier element. It allows cooling and heating a sample between −50 °C and 100 °C. The measurement chamber of the Lexsyg reader is equipped with a rotating arm to move aliquots between irradiation and readout position. For heating beyond 100 °C the sample is transported to the standard heating element of the reader. Performance tests of the developed cooling unit show that the samples can be held at temperatures below freezing (−40 °C) for more than 1 h and that the reproducibility of cooling and heating cycles is excellent. The new cooling unit enables irradiation and luminescence measurements below ambient temperatures in a fully automated setup.
1. Introduction Luminescence phenomena like other optoelectronic processes in solids, are strongly temperature dependent (e.g. McKeever and Chen, 1997). In spectroscopy for example, measurements at low temperatures help reducing noise and allow observations at high-resolution that would be blurred at ambient temperatures. In stimulated luminescence studies, cooling facilities have so far been restricted to research-grade equipment (e.g. Poolton et al., 2007) and not easily been available in automated systems that are used in dosimetry and dating applications. Here, we describe the design and the performance of a cooling system based on a Peltier element that is incorporated in the Lexsyg Research system (Richter et al., 2013). This cooling system extends the accessible temperature range from room temperature to −40 °C. Measurements above room temperature (up to 710 °C) are performed using the standard heating unit of the Lexsyg reader. Compared to He- or N- based cryostats such a design has a rather limited cooling performance but the ease of implementation allows routine low-temperature irradiation and luminescence measurement in a fully automated setup that would not be possible with a coolant based cryostat. The current setup allows peak
∗
cooling below −50 °C and stable holding of temperatures as low as −50 °C for more than 100 min. 2. Equipment design As platform for the new design a Lexsyg Research reader (Richter et al., 2013) is used. It comprises a sample storage wheel from which an aliquot is lifted up by a piston, picked up by a pneumatic grabber and transferred to the heater plate mounted on a sample arm. Rotating the arm allows the heater to access positions for irradiation (1.51 GBq, Sr90 beta ring source) and detection (Hamamatsu photomultiplier tubes: H7360-02 (300–650 nm) and H7421-50 (380–890 nm); Andor iXonUltra 897 EMCCD camera with thermoelectric cooling to −80 °C). The cooling unit is mounted on the rotating arm opposite the heater (see sketch in Fig. 1). This allows accessing irradiation and measurement positions as well as a transfer position to move a sample from the cooling element to the heater plate. The cooler is based on a thermoelectric cooling unit that transfers heat from one side of the device to the other with consumption of electrical energy (Peltier effect). The setup allows cooling a sample to a temperature of −50 °C and heating
Corresponding author. E-mail address: [email protected] (M. Discher).
https://doi.org/10.1016/j.radmeas.2019.02.017 Received 22 October 2018; Received in revised form 3 February 2019; Accepted 25 February 2019 Available online 01 March 2019 1350-4487/ © 2019 Elsevier Ltd. All rights reserved.
Radiation Measurements 124 (2019) 13–18
M. Discher, et al.
Fig. 2. Sample surface after cooling to −40 °C for 60 min (outer diameter of the stainless steel sample carrier depicted is 10 mm): (A) Sample carrier on the cooling plate as observed through the control position after cooling in lab atmosphere. Ice crystals are clearly visible as needle ice on the surface. Water ice originates from freezing air humidity. (B) Introducing a nitrogen atmosphere after evacuation of the measurement chamber significantly reduces ice crystal formation. (C) Further reduction of ice crystals using three consecutive cycles of evacuation and N2 flushing before cooling. Images (B) and (C) were taken using an EMCCD camera and a centrally focussing optics.
these temperatures can be held, (ii) the rates at which cooling and heating can be performed, and (iii) the overall repetitious accuracy of system. To closely mimic experimental conditions a blank sample carrier (cup) was used for all tests.
Fig. 1. Sketch view of the measurement chamber. The sample arm equipped with heating unit at one end and the cooling unit at the other end rotates between the positions for irradiation and detection. The transfer position is used to move samples between cooling and heating units of the arm.
3.1. Ice formation Cooling below zero degree C will result in ice forming on the cooled surfaces due to resublimation of air moisture even in air-conditioned laboratories were low levels of air humidity are maintained. Such ice formation will have detrimental effects due to loss of contact between sample and cooler, as well as unwanted absorption of light and radiation. When cooling the sample carrier to −40 °C for 60 min, ice growth on the disc surface is significant, hence impacting on luminescence experiments (Fig. 2A). To avoid ice formation the measurement chamber is evacuated and flushed with N2 (purity 99.999%) prior to cooling. This significantly reduces ice formation on the surface (Fig. 2B and C) to a negligible level.
up to 100 °C depending on current direction. For heating beyond 100 °C the sample is transported from the cooling element to the heating unit via a transfer position of the Lexsyg device. Full sample transport from the cooling to the heating device is resulting in a measurement interruption of approx. 2.5 min. In a Peltier element, cooling rate and temperature holding stability, as well as maximum cooling temperature rely on effective heat dissipation within the element and the transport away from it. To optimise the setup (i) the effective heat capacity was increased using a large copper mass on the rotating arm and (ii) the heat transfer out of the measurement chamber through the rotary feedthrough of sample arm was enhanced. Temperature is monitored using the specific resistance characteristic of four-point connected resistance thermometers (Pt1000 elements) mounted underneath the surface of the cooling position and on the backside of the Peltier element. Control of the cooling device is fully integrated in the electronics setup of the reader. It comprises of TEC05 Peltier driver from HEAD electronics that integrates a H-bridge driver and a controller chip. The TEC05 unit adjusts Peltier voltage and current to reach and stabilize target temperature. To optimize control performance low residual offset internally a proportional-integral controller is used. User access is through the operating software “LexStudio2.0” (Freiberg Instruments, 2018) allowing flexible control of temperature and timing as well as implementing pre-defined temperature-time curves for cooling and heating.
3.2. Minimum temperature and holding times Crucial parameters for cooling experiments are the minimum temperature the system can safely reach and the time such a temperature can be held as this limits the durations of irradiation and measurement. In the first set of experiments the empty sample carrier was cooled below ambient temperature within 100 s to a pre-defined temperature T (with T = 0, −20, −40, −45 and −50 °C) and keeping T constant for up to 3600 s. Fig. 3 shows the measured temperature and the pre-defined temperature curves for the five temperatures tested. The maximum holding time for T = 0, −20 and −40 °C is longer than the tested duration of 1 h. For T = −45 °C and T = −50 °C a small offset of 1 °C is recorded after 850 s and 140 s, respectively, beyond which a continuous increase of temperatures is observed.
3. Performance testing Tests were undertaken to check the general performance of the cooling element as well as the formation of ice on cooling unit and sample. Pre-defined temperature-time curves were set to test (i) the minimum temperature that can be reached and the maximum times
3.3. Cooling and heating rates To test the rate at which cooling and heating can be performed a second set of experiments was carried out: a temperature of 20 °C was
14
Radiation Measurements 124 (2019) 13–18
M. Discher, et al.
Fig. 3. Temperature holding target-performance comparison. Measured temperature (black symbols) and target temperature (red line) are plotted against time for cooling to 0, -20, −40, −45 and −50 °C. Full-data set is shown in the inset. For −45 and −50 °C the temperature increased after 850 and 140 s and experiment was terminated after 1000 s. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
held for 100 s before cooling to −40 °C at a pre-defined cooling rate β. After reaching −40 °C the temperature is held for 100 s before the sample is heated up to 20 °C using heating rate β. The tested cooling and heating rates β are 0.1, 0.5, 1.0 and 2.0 °C/s. Results are plotted in Fig. 4. The offset between target and measured values was calculated including the mean and standard deviation for the different heating rates β (Table 1). The performance tests show that for decreasing cooling and heating rates the offset decreases significantly. When temperature is kept constant the cooling unit shows less than 0.1 °C deviation from the
set temperature. Slow cooling and heating rates (i.e. 1 °C/s) are recommended to minimise the offset and possible thermal lag between nominal and sample temperature. 3.4. Surface temperature of a sample carrier To monitor the temperature at the sample a Pt100 temperature probe (temperature range −50 °C to 500 °C; precision of ± 0.10 °C at 0 °C (1/3 DIN)) was attached to sample carrier with conductive paste
15
Radiation Measurements 124 (2019) 13–18
M. Discher, et al.
Fig. 4. Cooling rate target-performance comparison: The measured temperatures are given as black symbols and the target temperature curve is displayed as red line. The inset gives the full data set including the heating to 20 °C. Different cooling and heating rates are tested to check the performance of the cooling unit. The tested rates are: 0.1, 0.5, 1.0 and 2.0 °C/s. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
polynomial fit of the data. The temperature difference is about 10% of the target temperature and can probably be explained by thermal lag due to (i) heat transfer through the stainless steel sample carrier (thickness 0.5 mm) and (ii) heat exchange with the atmosphere because measurements were carried out with an opened measurement chamber.
Table 1 Offset between target and measured values during cooling and heating using different cooling and heating rates β. Cooling/Heating rate β
Offset during cooling from 20 to −40 °C
Offset during heating from −40 to 20 °C
0.1 °C/s 0.5 °C/s 1.0 °C/s 2.0 °C/s
0.1 0.9 1.7 3.5
0.4 1.1 1.4 2.4
± ± ± ±
0.1 °C 0.4 °C 0.7 °C 1.3 °C
± ± ± ±
0.1 °C 0.3 °C 0.7 °C 1.0 °C
3.5. Repeatability test To test system performance after repeated cycles of cooling and heating, ten measurement cycles were repeated. For this test typical pre-defined temperature-time curves with different holding temperatures (T = −10, −20, −30, −40 and −50 °C) were implemented. The cooling and heating rates were always set to β = 1 °C/s. The overall reproducibility of system performance is fine (Fig. 6) showing very low standard deviation when temperatures are kept constant (δT < 0.1 °C). During the cooling and heating steps the standard deviation increases but typically remains between 1 and 2 °C. Previous tests showed that sudden fluctuations in temperature were related to temperature changes during N2 flushing of the measurement chamber (not shown). After adapting the operation software to check N2 pressure before a measurement and prevent N2 flushing during a measurement step such fluctuations disappeared.
and placed on the cooling unit. A 4-wire connection of the Pt100 probe was implemented to increase accuracy by compensating lead resistance of the connection lead. Temperatures were held for 100 s and the cooling rate β to reach the next temperature step was set to 1 °C/s. The temperature probe was read after 50 s and 100 s to allow for temperature stabilization. The response of the Pt100 probe is quick and measured values stabilise within view seconds. The difference between holding times of 50 s and 100 s are less than 0.1 °C. The average of both readings is given in Fig. 5a and plotted with the temperature recorded by the device. Temperature differences are plotted against temperature of the cooling unit and displayed in Fig. 5b together with a 2nd order
16
Radiation Measurements 124 (2019) 13–18
M. Discher, et al.
4. Summary and conclusions We present design and performance of a cooling unit integrated into a fully automated luminescence reader. A Peltier element is mounted in Lexsyg reader and enables cooling and heating a sample between −50 °C and 100 °C. Evacuating the measurement chamber and filling it with dry N2 to a slight over-pressure before cooling reduces ice formation to a negligible level and helps minimising potential effects on irradiation and luminescence read-out. System performance allows holding samples at temperatures below freezing (≥−40 °C) for more than 1 h. For temperatures below −45 °C the holding time is limited to several minutes. System performance is highly reproducible. Temperature control of the cooling unit is generally excellent but depends on cooling rate. For rates between 0.1 °C/s and 2 °C/s the absolute offset varies between 0.1 ± 0.1 °C and 3.5 ± 1.3 °C during cooling and between 0.4 ± 0.1 °C and 2.4 ± 1.0 °C during heating. For standard operations rates of 1 °C/s or slower are recommended. The overall reproducibility of system performance is fine and the temperature standard deviation stays below 2 °C during cooling and heating cycles verified by repeatability tests. The sample's temperature is about 10% higher of the given target temperature which is explained by thermal lag and has to be taken into account. For the very first time it is now possible to carry out irradiation and luminescence measurements below ambient temperatures in a fully automated setup. Acknowledgements Research of MD was partly conducted in the framework of EPU (Eurasia-Pacific UNINET) network and funded by the Federal Ministry of Science, Research and Economy (BMWFW) Austria (project period: 2017–2018). The authors thank Mag. Matthias Marbach for mounting the Pt100 temperature probe on the sample carrier and his assistance during the measurements.
Fig. 5. a) Temperature curve measured by the Peltier cooling unit (red curve) and the average surface temperature measured at the surface of the sample carrier (black symbols). b) Temperature differences plotted against temperature of the cooling unit. A polynomial fit (y = ax2+bx + c) with fitting parameter values of a = 1.705E-4 ± 0.545E-4, b = −0.056 ± 0.002 and c = 1.695 ± 0.034 describes the temperature differences. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Radiation Measurements 124 (2019) 13–18
M. Discher, et al.
Fig. 6. Test of repeatability for different pre-defined temperature-time curves. Each cooling and heating profile was repeated for 10 times. The standard deviation δT of the 10 temperatures measured at a given time is displayed as black symbols. The pre-set temperature curve is shown as red line. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
References
Poolton, N.R.J., Towlson, B.M., Hamilton, B., Wallinga, J., Lang, A., 2007. Micro-imaging synchrotron–laser interactions in wide band-gap luminescent materials. J. Phys. D Appl. Phys. 40 (12), 3557–3562. Richter, D., Richter, A., Dornich, K., 2013. Lexsyg – a new system for luminescence research. Geochronometria 40 (4), 220–228.
Freiberg Instruments, 10. September 2018. LexStudio 2.0 – User Friendly Operating Software. Available from: http://www.lexsyg.com/software/lexstudio-20operating-software.html. McKeever, S.W.S., Chen, R., 1997. Luminescence models. Radiat. Meas. 27, 625–661.
18
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Extending the measurement temperature range in a fully automated luminescence reader to −50 °C
T
Michael Dischera,∗, Kay Dornichb, Andreas Richterb, Barbara Mauza, Andreas Langa a b
Department of Geography and Geology, University of Salzburg, Hellbrunner Str. 34, 5020, Salzburg, Austria Freiberg Instruments GmbH, Delfter Str. 6, 09599, Freiberg, Germany
ARTICLE INFO
ABSTRACT
Keywords: Luminescence equipment development Cooling unit Novel instrumentation
It is today well established that the luminescence signal is temperature-dependent with compelling evidence provided for temperatures above room temperature. For the signal temperature dependence on lower temperatures, however, our knowledge is rather limited due to the lack of luminescence readers that allow cooling as well as heating in an automated setup. Here, we present technical details and test results of an upgraded Lexsyg Research luminescence reader equipped with a thermoelectric cooling device: a Peltier element. It allows cooling and heating a sample between −50 °C and 100 °C. The measurement chamber of the Lexsyg reader is equipped with a rotating arm to move aliquots between irradiation and readout position. For heating beyond 100 °C the sample is transported to the standard heating element of the reader. Performance tests of the developed cooling unit show that the samples can be held at temperatures below freezing (−40 °C) for more than 1 h and that the reproducibility of cooling and heating cycles is excellent. The new cooling unit enables irradiation and luminescence measurements below ambient temperatures in a fully automated setup.
1. Introduction Luminescence phenomena like other optoelectronic processes in solids, are strongly temperature dependent (e.g. McKeever and Chen, 1997). In spectroscopy for example, measurements at low temperatures help reducing noise and allow observations at high-resolution that would be blurred at ambient temperatures. In stimulated luminescence studies, cooling facilities have so far been restricted to research-grade equipment (e.g. Poolton et al., 2007) and not easily been available in automated systems that are used in dosimetry and dating applications. Here, we describe the design and the performance of a cooling system based on a Peltier element that is incorporated in the Lexsyg Research system (Richter et al., 2013). This cooling system extends the accessible temperature range from room temperature to −40 °C. Measurements above room temperature (up to 710 °C) are performed using the standard heating unit of the Lexsyg reader. Compared to He- or N- based cryostats such a design has a rather limited cooling performance but the ease of implementation allows routine low-temperature irradiation and luminescence measurement in a fully automated setup that would not be possible with a coolant based cryostat. The current setup allows peak
∗
cooling below −50 °C and stable holding of temperatures as low as −50 °C for more than 100 min. 2. Equipment design As platform for the new design a Lexsyg Research reader (Richter et al., 2013) is used. It comprises a sample storage wheel from which an aliquot is lifted up by a piston, picked up by a pneumatic grabber and transferred to the heater plate mounted on a sample arm. Rotating the arm allows the heater to access positions for irradiation (1.51 GBq, Sr90 beta ring source) and detection (Hamamatsu photomultiplier tubes: H7360-02 (300–650 nm) and H7421-50 (380–890 nm); Andor iXonUltra 897 EMCCD camera with thermoelectric cooling to −80 °C). The cooling unit is mounted on the rotating arm opposite the heater (see sketch in Fig. 1). This allows accessing irradiation and measurement positions as well as a transfer position to move a sample from the cooling element to the heater plate. The cooler is based on a thermoelectric cooling unit that transfers heat from one side of the device to the other with consumption of electrical energy (Peltier effect). The setup allows cooling a sample to a temperature of −50 °C and heating
Corresponding author. E-mail address: [email protected] (M. Discher).
https://doi.org/10.1016/j.radmeas.2019.02.017 Received 22 October 2018; Received in revised form 3 February 2019; Accepted 25 February 2019 Available online 01 March 2019 1350-4487/ © 2019 Elsevier Ltd. All rights reserved.
Radiation Measurements 124 (2019) 13–18
M. Discher, et al.
Fig. 2. Sample surface after cooling to −40 °C for 60 min (outer diameter of the stainless steel sample carrier depicted is 10 mm): (A) Sample carrier on the cooling plate as observed through the control position after cooling in lab atmosphere. Ice crystals are clearly visible as needle ice on the surface. Water ice originates from freezing air humidity. (B) Introducing a nitrogen atmosphere after evacuation of the measurement chamber significantly reduces ice crystal formation. (C) Further reduction of ice crystals using three consecutive cycles of evacuation and N2 flushing before cooling. Images (B) and (C) were taken using an EMCCD camera and a centrally focussing optics.
these temperatures can be held, (ii) the rates at which cooling and heating can be performed, and (iii) the overall repetitious accuracy of system. To closely mimic experimental conditions a blank sample carrier (cup) was used for all tests.
Fig. 1. Sketch view of the measurement chamber. The sample arm equipped with heating unit at one end and the cooling unit at the other end rotates between the positions for irradiation and detection. The transfer position is used to move samples between cooling and heating units of the arm.
3.1. Ice formation Cooling below zero degree C will result in ice forming on the cooled surfaces due to resublimation of air moisture even in air-conditioned laboratories were low levels of air humidity are maintained. Such ice formation will have detrimental effects due to loss of contact between sample and cooler, as well as unwanted absorption of light and radiation. When cooling the sample carrier to −40 °C for 60 min, ice growth on the disc surface is significant, hence impacting on luminescence experiments (Fig. 2A). To avoid ice formation the measurement chamber is evacuated and flushed with N2 (purity 99.999%) prior to cooling. This significantly reduces ice formation on the surface (Fig. 2B and C) to a negligible level.
up to 100 °C depending on current direction. For heating beyond 100 °C the sample is transported from the cooling element to the heating unit via a transfer position of the Lexsyg device. Full sample transport from the cooling to the heating device is resulting in a measurement interruption of approx. 2.5 min. In a Peltier element, cooling rate and temperature holding stability, as well as maximum cooling temperature rely on effective heat dissipation within the element and the transport away from it. To optimise the setup (i) the effective heat capacity was increased using a large copper mass on the rotating arm and (ii) the heat transfer out of the measurement chamber through the rotary feedthrough of sample arm was enhanced. Temperature is monitored using the specific resistance characteristic of four-point connected resistance thermometers (Pt1000 elements) mounted underneath the surface of the cooling position and on the backside of the Peltier element. Control of the cooling device is fully integrated in the electronics setup of the reader. It comprises of TEC05 Peltier driver from HEAD electronics that integrates a H-bridge driver and a controller chip. The TEC05 unit adjusts Peltier voltage and current to reach and stabilize target temperature. To optimize control performance low residual offset internally a proportional-integral controller is used. User access is through the operating software “LexStudio2.0” (Freiberg Instruments, 2018) allowing flexible control of temperature and timing as well as implementing pre-defined temperature-time curves for cooling and heating.
3.2. Minimum temperature and holding times Crucial parameters for cooling experiments are the minimum temperature the system can safely reach and the time such a temperature can be held as this limits the durations of irradiation and measurement. In the first set of experiments the empty sample carrier was cooled below ambient temperature within 100 s to a pre-defined temperature T (with T = 0, −20, −40, −45 and −50 °C) and keeping T constant for up to 3600 s. Fig. 3 shows the measured temperature and the pre-defined temperature curves for the five temperatures tested. The maximum holding time for T = 0, −20 and −40 °C is longer than the tested duration of 1 h. For T = −45 °C and T = −50 °C a small offset of 1 °C is recorded after 850 s and 140 s, respectively, beyond which a continuous increase of temperatures is observed.
3. Performance testing Tests were undertaken to check the general performance of the cooling element as well as the formation of ice on cooling unit and sample. Pre-defined temperature-time curves were set to test (i) the minimum temperature that can be reached and the maximum times
3.3. Cooling and heating rates To test the rate at which cooling and heating can be performed a second set of experiments was carried out: a temperature of 20 °C was
14
Radiation Measurements 124 (2019) 13–18
M. Discher, et al.
Fig. 3. Temperature holding target-performance comparison. Measured temperature (black symbols) and target temperature (red line) are plotted against time for cooling to 0, -20, −40, −45 and −50 °C. Full-data set is shown in the inset. For −45 and −50 °C the temperature increased after 850 and 140 s and experiment was terminated after 1000 s. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
held for 100 s before cooling to −40 °C at a pre-defined cooling rate β. After reaching −40 °C the temperature is held for 100 s before the sample is heated up to 20 °C using heating rate β. The tested cooling and heating rates β are 0.1, 0.5, 1.0 and 2.0 °C/s. Results are plotted in Fig. 4. The offset between target and measured values was calculated including the mean and standard deviation for the different heating rates β (Table 1). The performance tests show that for decreasing cooling and heating rates the offset decreases significantly. When temperature is kept constant the cooling unit shows less than 0.1 °C deviation from the
set temperature. Slow cooling and heating rates (i.e. 1 °C/s) are recommended to minimise the offset and possible thermal lag between nominal and sample temperature. 3.4. Surface temperature of a sample carrier To monitor the temperature at the sample a Pt100 temperature probe (temperature range −50 °C to 500 °C; precision of ± 0.10 °C at 0 °C (1/3 DIN)) was attached to sample carrier with conductive paste
15
Radiation Measurements 124 (2019) 13–18
M. Discher, et al.
Fig. 4. Cooling rate target-performance comparison: The measured temperatures are given as black symbols and the target temperature curve is displayed as red line. The inset gives the full data set including the heating to 20 °C. Different cooling and heating rates are tested to check the performance of the cooling unit. The tested rates are: 0.1, 0.5, 1.0 and 2.0 °C/s. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
polynomial fit of the data. The temperature difference is about 10% of the target temperature and can probably be explained by thermal lag due to (i) heat transfer through the stainless steel sample carrier (thickness 0.5 mm) and (ii) heat exchange with the atmosphere because measurements were carried out with an opened measurement chamber.
Table 1 Offset between target and measured values during cooling and heating using different cooling and heating rates β. Cooling/Heating rate β
Offset during cooling from 20 to −40 °C
Offset during heating from −40 to 20 °C
0.1 °C/s 0.5 °C/s 1.0 °C/s 2.0 °C/s
0.1 0.9 1.7 3.5
0.4 1.1 1.4 2.4
± ± ± ±
0.1 °C 0.4 °C 0.7 °C 1.3 °C
± ± ± ±
0.1 °C 0.3 °C 0.7 °C 1.0 °C
3.5. Repeatability test To test system performance after repeated cycles of cooling and heating, ten measurement cycles were repeated. For this test typical pre-defined temperature-time curves with different holding temperatures (T = −10, −20, −30, −40 and −50 °C) were implemented. The cooling and heating rates were always set to β = 1 °C/s. The overall reproducibility of system performance is fine (Fig. 6) showing very low standard deviation when temperatures are kept constant (δT < 0.1 °C). During the cooling and heating steps the standard deviation increases but typically remains between 1 and 2 °C. Previous tests showed that sudden fluctuations in temperature were related to temperature changes during N2 flushing of the measurement chamber (not shown). After adapting the operation software to check N2 pressure before a measurement and prevent N2 flushing during a measurement step such fluctuations disappeared.
and placed on the cooling unit. A 4-wire connection of the Pt100 probe was implemented to increase accuracy by compensating lead resistance of the connection lead. Temperatures were held for 100 s and the cooling rate β to reach the next temperature step was set to 1 °C/s. The temperature probe was read after 50 s and 100 s to allow for temperature stabilization. The response of the Pt100 probe is quick and measured values stabilise within view seconds. The difference between holding times of 50 s and 100 s are less than 0.1 °C. The average of both readings is given in Fig. 5a and plotted with the temperature recorded by the device. Temperature differences are plotted against temperature of the cooling unit and displayed in Fig. 5b together with a 2nd order
16
Radiation Measurements 124 (2019) 13–18
M. Discher, et al.
4. Summary and conclusions We present design and performance of a cooling unit integrated into a fully automated luminescence reader. A Peltier element is mounted in Lexsyg reader and enables cooling and heating a sample between −50 °C and 100 °C. Evacuating the measurement chamber and filling it with dry N2 to a slight over-pressure before cooling reduces ice formation to a negligible level and helps minimising potential effects on irradiation and luminescence read-out. System performance allows holding samples at temperatures below freezing (≥−40 °C) for more than 1 h. For temperatures below −45 °C the holding time is limited to several minutes. System performance is highly reproducible. Temperature control of the cooling unit is generally excellent but depends on cooling rate. For rates between 0.1 °C/s and 2 °C/s the absolute offset varies between 0.1 ± 0.1 °C and 3.5 ± 1.3 °C during cooling and between 0.4 ± 0.1 °C and 2.4 ± 1.0 °C during heating. For standard operations rates of 1 °C/s or slower are recommended. The overall reproducibility of system performance is fine and the temperature standard deviation stays below 2 °C during cooling and heating cycles verified by repeatability tests. The sample's temperature is about 10% higher of the given target temperature which is explained by thermal lag and has to be taken into account. For the very first time it is now possible to carry out irradiation and luminescence measurements below ambient temperatures in a fully automated setup. Acknowledgements Research of MD was partly conducted in the framework of EPU (Eurasia-Pacific UNINET) network and funded by the Federal Ministry of Science, Research and Economy (BMWFW) Austria (project period: 2017–2018). The authors thank Mag. Matthias Marbach for mounting the Pt100 temperature probe on the sample carrier and his assistance during the measurements.
Fig. 5. a) Temperature curve measured by the Peltier cooling unit (red curve) and the average surface temperature measured at the surface of the sample carrier (black symbols). b) Temperature differences plotted against temperature of the cooling unit. A polynomial fit (y = ax2+bx + c) with fitting parameter values of a = 1.705E-4 ± 0.545E-4, b = −0.056 ± 0.002 and c = 1.695 ± 0.034 describes the temperature differences. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
17
Radiation Measurements 124 (2019) 13–18
M. Discher, et al.
Fig. 6. Test of repeatability for different pre-defined temperature-time curves. Each cooling and heating profile was repeated for 10 times. The standard deviation δT of the 10 temperatures measured at a given time is displayed as black symbols. The pre-set temperature curve is shown as red line. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
References
Poolton, N.R.J., Towlson, B.M., Hamilton, B., Wallinga, J., Lang, A., 2007. Micro-imaging synchrotron–laser interactions in wide band-gap luminescent materials. J. Phys. D Appl. Phys. 40 (12), 3557–3562. Richter, D., Richter, A., Dornich, K., 2013. Lexsyg – a new system for luminescence research. Geochronometria 40 (4), 220–228.
Freiberg Instruments, 10. September 2018. LexStudio 2.0 – User Friendly Operating Software. Available from: http://www.lexsyg.com/software/lexstudio-20operating-software.html. McKeever, S.W.S., Chen, R., 1997. Luminescence models. Radiat. Meas. 27, 625–661.
18