The number of ultra-short, ultra-intense laser devices has been increasing worldwide since the 1970s. Such devices can produce lasers with intensity up to 1021 W cm−2 on subpicosecond (ps) timescales. By focusing an ultra-short, ultra-intense laser to a diameter scale of tens of microns and shining it on a high-Z thick target (such as gold, copper, and tantalum), extremely high radiation doses can be generated, which can provide extreme situations for physics research (Nuckolls et al. 1972; Schnurer et al. 1995; Guo et al. 2001; Chen et al. 2004; Courtois et al. 2009; Courtois et al. 2011; Daido et al. 2012). When a beam of ultra-short, ultra-intense laser irradiates a solid target, the atoms on the target surface will be stripped into a high-charge state and generate high-temperature high-density plasma (Tajima and Dawson 1979; Kruer and Estabrook 1985; Brunel 1987; Wilks et al. 1992; Pukhov et al. 1999; Faure et al. 2004; Geddes et al. 2004; Mangles et al. 2004; Malka et al. 2006; Kim et al. 2013). Most laser energy will then be absorbed in the plasma and used to produce relativistic hot electrons, which will further generate x rays by ionization or bremsstrahlung. Some photons with high energy could produce outwardly emitted neutrons and positrons through photonuclear reactions and electron pair effects (Glinec et al. 2005; Chen et al. 2010; Courtois et al. 2011; Macchi et al. 2013; Pomerantz et al. 2014; Yang et al. 2014; Brenner et al. 2015). In addition, due to the charge separation, a strong electrostatic field will be formed near the back of the target, and the protons on the target surface will be accelerated to emit outward (Daido et al. 2012; Macchi et al. 2013; Wagner et al. 2016).
Due to the emission of these particles, ultra-short, ultra-intense laser devices could generate a relatively high ionizing radiation dose up to tens of mSv in tens of picoseconds, resulting in a high instantaneous dose rate of about 107 Sv s−1 (Schnurer et al. 1995; Guo et al. 2001; Chen et al. 2004; Courtois et al. 2009, 2011), which poses a serious challenge to the radiation protection of ultra-short, ultra-intense devices. Given the radiation dose generated by ultra-short, ultra-intense laser facilities, researchers have carried out several measurement studies. In 2015, Liang and colleagues measured the photon dose generated by the interaction between solid targets and lasers with an intensity of 1 × 1018 ~ 7.1 × 1019 W cm−2. It was found that almost all the active dosimeters available in the market, including ionization chambers, could not work properly in the radiation field with such an extremely high dose rate. The output values of active detectors were much smaller than that of passive detectors such as thermoluminescence dosimeter (TLD) detectors, which were considered to be accurate (Liang et al. 2016). In 2016, Yang and colleagues measured the photon dose generated by an “XG-III” laser device using TLD detectors, optically stimulated luminescence (OSL) detectors, and 451P ionization chambers, and the results indicated that the ionization chambers underestimate the surrounding dose by one to two orders of magnitude compared to the TLD and OSL detectors (Yang et al. 2017a; Yang 2018).
To ensure the radiation safety of the staff in the vicinity, a real-time radiation dose measurement is necessary. Due to the low saturation threshold and slow response, charge carriers produced in most active detectors undergo a recombination process, leading to a decline in the number of charge carriers inside (Laitano et al. 2006). Consequently, the dose measurement results will be underestimated.
To reduce the recombination losses of charge carriers, a high saturation threshold of the detector is necessary. Compared with conventional active detector materials like silicon, diamond has a higher bandgap, higher carrier mobility, and stronger C-C bond (Wort et al. 2008), as shown in Table 1. Because of these characteristics, diamond detectors show better performance than traditional active detectors with advantages in many parts, such as lower leakage current, faster response, and higher saturation threshold (Abdel-Rahman et al. 2012). Considering these merits, diamond is expected to be fit for the real-time dose measurement in ultra-short, ultra-intense pulsed radiation fields.
Table 1 -
Comparison of characteristics between diamond and single crystal silicon.
||Single crystal silicon
|Band gap (eV)
|Resistivity (Ω × cm)
|Breakdown electric field strength (V cm−1)
|Electron mobility (cm2 V−1 s−1)
|Hole mobility (cm2 V−1 s−1)
|Saturation constant (μm ns−1)
|Energy required to generate an electron-hole pair (eV)
Previous researchers have investigated the performance of chemical vapor deposition (CVD) diamond detectors under different circumstances. In 2004, H. Frais-Kölbl and colleagues tested the performance of a CVD diamond detector using a proton beam generated by the cyclotron facility at Indiana University. The detector was exposed to beam energies of 200, 150, 105, 75, and 55 MeV. They recorded 50-ns-long sequences of detector signals and found that the average rise time of the signal generated by the diamond detector was 350 ps, and the detection efficiency was 99.9%. They assume that the diamond detector can be used as an ultrafast particle counter for proton detection (Frais-Kolbl et al. 2004). In 2010, L. Ryć and others adopted a CVD diamond detector to measure the soft x rays generated by the Prague Asterix Laser System (PALS). The results showed that the detector had good sensitivity to fast ions and soft x rays with an energy of 0.1 to 9 keV. The diamond detector was considered to be usable for the measurement of Z-pinch soft x ray produced in 2 μs (Ryc et al. 2010). In 2012, Yu and others used the 8 ps laser system and the “SG-III” prototype device to study the performance of the CVD diamond detector. It was found that the rise time of the signal was 175 ps, and the overall response time was 444 ps (Yu et al. 2013; Zheng et al. 2016).
In this research, an active dose measurement device was developed based on a CVD diamond detector. Its dose rate response to photons with different energy was calibrated using the x-ray sources of the China National Institute of Metrology for x ray with energy lower than 250 keV. For high-energy x rays, the dose rate response of the device was obtained by simulation methods. The simulation and experimental results in the low-energy range were used to obtain the charge collection efficiency, which was then used to correct the dose rate response in the high-energy range.
The feasibility of real-time photon dose measurement in pulsed radiation fields generated by high-intensity laser devices was investigated using the “SG-II” laser facility. The doses given by this dose measurement device were close to those of TLD detectors, which preliminarily verified the feasibility of using this measurement device for real-time photon dose measurement.
MATERIAL AND METHODS
CVD diamond detector
Our dose measurement device was developed based on a CVD diamond detector and self-designed electronic circuits. The CIVIDEC Instrumentation has rich experience in manufacturing CVD diamond detectors, and its detectors have many practical applications in the field of physics research including the Large Hadron Collider (LHC) (Griesmayer et al. 2012; Xu et al. 2016). Specifically, the CVD diamond detector (model: B1-HV) provided by CIVIDEC was selected to develop our dose measurement device. Its leakage current is only about 100 fA under a negative bias voltage of 140 V. The size of the B1-HV CVD diamond used here is 4.5 mm × 4.5 mm × 0.14 mm, while the gold electrodes on both sides are 4 mm × 4 mm. The diamond detector is placed inside an aluminum housing box with dimensions of 55 mm × 55 mm × 15 mm, and the thickness of this shell is about 1 mm.
Electronics and data acquisition
As the distance between two electrodes is only 140 μm, the detector could output a large current signal in a short time, which may exceed the measurement range of the electrometer. Consequently, the electronic circuits of the dose measurement device need to be designed to achieve the purpose of decreasing the instantaneous output current. A pre-integration circuit composed of a capacitor and a resistor was designed in electronic circuits, as shown in Fig. 2. The charges collected by the electrodes of the diamond detector could be stored in the capacitor and gradually released. Through this pre-integration circuit, the instantaneous current could be reduced with the amount of the charge unchanged. The signal is then amplified by a current-sensitive preamplifier. Afterward, an analog to digital converter (ADC) is employed to digitize the signal.
Evaluation of the dose rate response
In this research, the dose rate response of the measurement device was defined as the ratio of the output current to the surrounding air kerma rate (air kerma means the amount of energy that is transferred from photons to electrons per unit mass at a certain position in air), and this value varied with the energy of photons. Limited by the photon energy of the x-ray source, the dose rate response was obtained in two ranges: low energy (<250 keV) and high energy.
Low-energy x-ray response
In terms of low-energy x rays, the dose rate response of the measurement device could be obtained directly from experiments. Experiments were carried out at the China National Institute of Metrology where the x-ray generator could generate x rays with energy from 39 keV to 250 keV, which could produce an air kerma rate up to 10 Gy h−1.
The x-ray source provided by the China National Institute of Metrology has been calibrated to obtain different air kerma rates at a certain position by adjusting the current of the x-ray tube. In the case of a known air kerma rate, the measurement device was placed at the calibration position, and its output current was recorded. By fitting the data of air kerma rate and output current, it could be found out whether there was a linear relationship between these two quantities, and a relevant parameter “linearity” could be obtained to evaluate the linearity of the dose rate response.
In the experiment, the dose measurement device was placed at the calibration position, and the leakage current of the device was recorded as IEB while the x-ray source was off. After the x-ray source was turned on, the output current was recorded as IES, and the net current IE could be calculated as IE = IES − IEB. The air kerma rate KE, which had been calibrated at this location before, was already known. The dose rate response of the measurement device to the photons with energy E is:
At a fixed energy E, the Ordinary Least Squares (OLS) method was used to fit a straight line of IE ~ KE. The vertical distances (ΔIE) between the data points and the fitted straight line were calculated for different data points. Then the maximum distance ΔIEmax was compared with the maximum current IEmax to evaluate the linearity η, which is defined as η = ΔIEmax / IEmax × 100%.
High-energy x-ray response
In the high-energy range, only the 137Cs source was available for experiments, so simulations were carried out as a supplement. In the simulation, the measurement device was located 1 m away from an isotropic photon source with an energy of 15 keV to 5 MeV. Monte Carlo program FLUKA (ver 2011.2x) was used to get the energy deposited in the diamond detector and the surrounding air kerma (Ferrari et al. 2005). Since the amount of the charge generated in the diamond detector can be obtained using the energy required to generate an electron-hole pair, the dose rate response can be calculated with these simulation results:
where RE, QE, and KE represent the simulated dose rate response, the charge generated in the diamond, and the air kerma around the diamond at x-ray energy E, respectively. These simulation results are all normalized to each primary particle.
Since the dose rate response obtained above does not take the impact of the detector's charge collection efficiency into account, it is necessary to obtain the charge collection efficiency and modify the simulated dose rate response (Wang 2008). The simulation and experimental results of low-energy x rays were compared to get the charge collection efficiency and modify the results in the high energy range. To verify the feasibility of using the modified simulation results in the high-energy range, experiments were carried out using the 137Cs source. The modified simulated dose rate response was compared with the corresponding experimental result.
Performance evaluation on laser facility
Despite the fast response of diamond detectors, its feasibility of measuring radiation dose generated by ultra-short, ultra-intense laser devices required further research. The experiment was carried out using the “SG-II” laser device, a Nd:Glass laser device, which could provide a laser with power up to 150 TW (Fan et al. 2018). During the experiment, five shots were fired (#011, #012, #013, #014, and #015). The target was gold with a thickness of 2 mm. The wavelength of the laser is 1,052 nm. The energy of the laser irradiated on the target surface was about 150 J, and the size of the focal spot on the target surface is 35 μm × 35 μm. The typical laser parameters during the experiment consisted of a pulse duration of 2 ps and a laser intensity of 1.22 × 1019 W cm−2, which could produce a large dose rate.
Though the radiation field generated by the interaction between the high-intensity laser and the solid target is composed of different kinds of particles, with the shielding effect of the wall of the target chamber, most electrons and protons could be blocked. Previous studies show that there were far more photons than other particles outside the target chamber. Some researchers measured the fluence of different kinds of particles and found the fluence of photons was about three orders of magnitude higher than that of neutrons (Liang et al. 2016; Yang et al. 2017b). By simulating the energy deposited by different particles in the detector, it could be found that at the same energy and the same fluence per area, photons could deposit more energy inside the diamond detector, and each photon can deposit 1 to 5 times as much energy as the neutron deposits. In addition, the diamond detector used in this experiment was not equipped with neutron converters such as lithium and boron. In summary, the contribution of neutrons in this experiment was much smaller than that of photons. Therefore, in the dose-rate response of diamond detectors, only photons with different amounts of energy were considered.
To use this dose measurement device to measure the radiation dose around the “SG-II” laser device, the dose rate response of the measurement device is required. The spectra of photons emitted were measured by the filter stack spectrometer and then used as the source term of simulation to get the dose rate response. With the charge collection efficiency obtained before, the simulation results could be used to get the dose rate response of the measurement device in the experimental environment.
During the experiment, the output current of the dose measurement device was measured and converted to the surrounding air kerma using the dose-rate response obtained before. In the meanwhile, the surrounding air kerma was also measured by the TLD detectors at the same location. Fig. 3 depicts the layout of the experimental equipment. Photon spectra were measured using a filter stack spectrometer. Along the laser incidence direction, the filter stack spectrometer, the dose measurement device, and a set of TLD detectors were arranged. A photograph of the measurement device was shown in Fig. 4.
RESULTS AND DISCUSSION
Low-energy x-ray response
By adjusting the operating parameters of the x-ray tube, the dose rate response of the measurement device to photons with different amounts of energy was obtained. At a certain energy, we can obtain different air kerma rates by adjusting the current of the x-ray tube. Under the given energy and air kerma rate, we recorded the output current of the dose measurement device so as to obtain the measurement results consisting of air kerma rate and output current.
X-ray sources were with energy amounts of 39, 60, 80, 129, 164, and 208 keV, respectively. In normal conditions, the B1-HV diamond detector works under a negative bias voltage of 140 V, and the leakage current generated is about 0.17 pA. Considering the subsequent electronics, the output current data could be considered accurate and reliable when it ranges from 0.1 pA to 3 μA.
Fig. 5 shows the dose rate dependence of the diamond detector to photons with energy of 39, 60, 80, 129, 164, and 208 keV, respectively.
To verify that there is a linear relationship between the output current and air kerma rate, we fitted the results using the OLS method and plotted the fitted curve in the figure. The fitting results show that R2 is greater than 0.99 at all energies, showing good linearity relationship between the air kerma rate and the output current of the detector. According to the conclusion raised by Ade et al. (2014), there exists a relationship between the electrical conductivity of the diamond detector σ and the dose rate around (Fowler 1966), in which Δ is the linearity index, a parameter accounting for the dose rate dependence that could be obtained by data fitting. As the output current I is proportional to the conductivity σ, the relationship between the output current I and the dose rate around D˙r could be expressed as: . In the range of photon energy involved in the experiment, the dose rate D˙r and the air kerma rate K˙r are almost equal. Therefore, the following relationship exists between the output current and the surrounding air kerma rate : .
Besides the linear function, the measured data could also be fitted to the power function to obtain the relationship between the output current I and the air kerma rate K˙r, and the results are shown in the Table 2. It could be found that in the case of x-ray irradiation with the energy of 39 keV ~ 208 keV, the Δ values are very close to 1 and are located in the range of Δ values (0.79 ~ 1.03) given by Ade et al. (2014), which indicates that the measurement device has a linear dose rate response. In Table 2, the Δ values are larger than 1 at some energy values, which can be explained by the nature of the traps in the diamond crystal. Fowler expounds that if the traps in the crystal have different capture cross sections, the Δ value may exceed 1 (Fowler 1966). In addition, Abdel-Rahman et al. (2012) expound that if the traps in the crystal are non-uniformly distributed, the value of Δ may also exceed 1.
Table 2 -
Dose rate response and linearity of the diamond detector.
||Dose rate response (pAh mGy−1)
||Linearity η (%)
To better evaluate the dose rate response of the measurement device, the linearity η of the linear relationship is shown in Table 2. According to the experimental results, the dose measurement device has good linearity in the range of 3.39 mGy h−1 to 10.58 Gy h−1, with no significant saturation phenomenon found.
High-energy x-ray response
In the Monte Carlo simulation, the measurement device was placed 1 m away from the photon source. The charge generated in the diamond and the air kerma near the diamond were simulated and recorded during simulation. The dose rate response result is given by eqn (2). Fig. 6 shows the simulated dose rate response of the dose measurement device to photons with different energies.
The simulation and experimental results in the low-energy range were compared to get the charge collection efficiency, which was then used to modify the simulated dose-rate response in the high energy range.
The uncertainty of the measurement results of the dose measurement device is composed of two parts: one is the statistical uncertainty, and the other is the systematic uncertainty. Since the dose measurement device is coupled by the diamond detector and electronics, the uncertainty of the diamond detector needs to be further determined by the manufacturer, in this study, the system uncertainty cannot be considered for the time being; only the statistical uncertainty can be considered. In FLUKA simulation, the number of particles we simulated reached 2E9, so the error of FLUKA simulation results was minimal. Among them, the error of energy deposited in diamond is less than 0.05%, while the error of energy deposited in surrounding air is less than 0.1%. Therefore, the influence of the Monte Carlo simulation result is not considered here. In summary, the uncertainty considered here only comes from the statistical uncertainty of the dose measurement device. However, in experiment, due to the relatively stable current output by the diamond detector, the statistical uncertainty of the output current is only about 1%, resulting in a very low uncertainty of dose rate response after uncertainty transmission, so it is not shown in Fig. 7.
The comparison between the simulated and experimental results shows that the charge collection efficiency of this dose measurement device is 80.3%. Considering the charge collection efficiency, the simulated and experimental results in the low energy range are shown in Fig. 7. This charge collection efficiency is close to the value of 70% to 97% measured by Sato and colleagues (Sato et al. 2016).
To verify the feasibility of applying the charge collection efficiency measured in the low-energy range to the high-energy range, an experiment was carried out using the 137Cs source. In the experiment, the dose rate generated by the 137Cs source was in the range of 6.07 ~ 157.30 mGy h−1. The simulated and measured dose rate responses were 0.22 and 0.24, respectively. The deviation between these two values was only 8.3%. Since the average energy of x ray produced by the ultra-short, ultra-intense laser device is about 700 keV, which is close to that of 137Cs source, it could be considered that the simulated dose-rate response is suitable for dose measurement in the environment of a laser device after modification.
Measurement in the China Academy of Engineering Physics
While experimenting on the laser facilities, the photon spectra is different for each shot due to the instability of the laser pulse width and peak energy, even though the target material and other major parameters remain unchanged. To obtain a more accurate energy response for the dose measurement device, we measured the spectra of different shots in the experiment. The filter stack spectrometers used in the experiments employ some image plates sandwiched between several thin-foil filters. With the filtration of the filters, the rear image plates are more sensitive to high-energy photons than the front ones (Yu et al. 2017). Based on the energy response function and the depth signal curves recorded by these image plates, an unfolding procedure was employed to acquire the photon spectrum. Fig. 8 shows the photon spectra of different shots measured by the filter stack spectrometer in the incidence direction of the laser.
With the photon spectra measured, the energy response of the dose measurement device could be simulated by the Monte Carlo program FLUKA. Specifically, we established a model of the “SG-II (upgraded)” laser device. The position of the dose measurement device replicates the arrangement of the experiment, and the photon spectra measured in the experiment were used as the source term of simulation. Using the charge collection efficiency obtained before, the response of the dose measurement device in the experiment could be obtained by simulating the charge generated in the diamond and the air kerma around the diamond.
Table 3 shows the dose-rate response of the dose measurement device for different shots in the experiment; the surrounding radiation dose could be obtained based on these values.
Table 3 -
Efficiency corrected response for each laser shot.
||Efficiency corrected response [pA/(mGy h−1)]
Dose measurement results
During the experiment, the air kerma outside the target chamber was measured by the measurement device and TLD detectors simultaneously. The TLD detectors used in the experiment were calibrated using a 137Cs radiation source of the China National Institute of Metrology, whose air kerma measurement range is from 10 μGy to 10 Gy. In terms of the measurement device, its output current was recorded, and then the air kerma around could be obtained using the calibrated dose rate response.
The uncertainty of the measurement results of TLD is composed of two parts: one is the statistical uncertainty, and the other is the systematic uncertainty. In every single shot, three TLDs were used for measurement at the same time, and the standard deviation of the three measurement results was used as the statistical uncertainty. As for the systematic uncertainty, the main sources are calibration uncertainty, energy response uncertainty, nonlinear uncertainty, and angular response uncertainty. In this experiment, the calibration uncertainty is 5.4%. From the x-ray dose and average photon energy in this experiment, it can be estimated that the nonlinear uncertainty is about 2%, and the energy response uncertainty is about 2%. In addition, according to previous experiments, the angular response uncertainty is about 3% in the 0° ~ 90° direction. For the dose measurement device, since the systematic uncertainty was not calibrated in the calibration experiment and the same measurement cannot be repeated in the experiment, its uncertainty is not given here.
After considering the statistical error during experiments, it can be seen from Table 4 and Figs. 9 and 10 that the results given by the dose measurement device agree well with those by TLD detectors, and the deviation is less than 20%, which verifies the feasibility of using the diamond detector to measure the radiation dose generated by ultra-short, ultra-intense laser devices.
Table 4 -
Comparison of air kerma given by the dose measurement device and TLD detectors.
||Air kerma around (μGy)
||Charge generated (C)
||Experimental response [pA/(mGy h−1)]
||Dose measurement device
||1.30 × 10−11
||6.32 × 10−11
||7.06 × 10−11
||8.47 × 10−11
||1.16 × 10−10
In the experiments, despite the consideration of the statistical error, there is still some bias between the results given by the dose measurement device and those by TLD detectors. This may be due to a variety of factors, including the bias of dose rate response, deviations in experiments, and scattering caused by experimental components. First, validation experiments on the 137Cs source show that the modified dose rate response given by simulation has a deviation of less than 10% from the real value. Considering the complexity of the photon spectra generated by the laser device, there may be a deviation between the modified dose-rate response and the real value, which may be one of the factors. Second, in the experiment, the position of the dose measurement device may not be aligned with the direction of laser emission accurately. According to the previous research of Yang and others, the energy spectrum generated by laser facilities has angular inhomogeneity (Yang et al. 2017a). Therefore, the simulated dose rate response may deviate from the actual value, resulting in deviations in the results. Third, there are a large number of experimental pipelines and other auxiliary facilities inside and around the “SG-II” laser device. These components could produce some scattering photons, which could lead to deviations. Fourth, the uncertainty of this dose measurement device has not been accurately evaluated, so the error in the measurement results is inevitable. In addition, some researchers also pointed out that in an intense radiation field, the diamond detector may suffer radiation damage (Campbell and Mainwood 2000), which may also be one of the reasons for the deviation of the experimental results. The above contents need further research in subsequent experiments.
A prototype dose measurement device was developed based on a CVD diamond detector. Its capability of measuring the radiation dose generated by the ultra-short, ultra-intense laser facilities was characterized using Monte Carlo simulation and experiments.
First, the dose rate responses of the dose measurement device to photons with different energy were acquired with simulation and experiments. The experimental results were obtained for x ray with energy lower than 250 keV, which were then applied to get the charge collection efficiency of this dose measurement device. Results show that the diamond detector has no saturation phenomenon in the dose rate range of 3.39 mGy h−1 to 10.58 Gy h−1 and has a good linear performance. The charge collection efficiency is about 80%, which is close to that of similar products.
Second, some experiments were carried out at the “SG-II (upgraded)” laser device to study the feasibility of real-time dose measurement of ultra-short, ultra-intense laser devices. Photon spectra, the output current of the diamond detector, and the surrounding air kerma were measured simultaneously. The photon spectra measured were used to simulate the dose-rate response in the experimental circumstance. The TLD detectors and the dose measurement device give the air kerma values around, respectively. The air kerma values given by the dose measurement device are in good agreement with that of TLD detectors, and the feasibility of using the diamond detector to measure the radiation dose of the ultra-short, ultra-intense laser device was preliminarily verified.
As for the neutron dose measurement, diamond detectors do have some potential. To improve the capability of diamond detectors in neutron measurement, neutron converters such as lithium and boron can be added to the dose measurement device. It should be pointed out that when this device is used for the measurement of the dose produced by the ultra-short, ultra-intense laser device, its energy response needs to be fully considered because the accuracy of the measurement results depends on it. Our previous study shows that different target compositions have different cross sections for particle emissions (Yang et al. 2017c). Our results show that when the target material is a high-Z thick target, compared with a thin target, high-energy photons account for a larger proportion of the outgoing photons; thus, the response of the detector needs to be adjusted according to the different target materials selected in practical operation.
In addition, for the dose measurement of the ultra-intense pulsed radiation field, it is still necessary to increase the research intensity of the new detector materials and strive to achieve real-time dose measurement to provide reference for radiation protection work. In subsequent research, more experiments are going to be carried out based on different laser devices, and more data points will be added to better verify the practicability of the diamond detector.
This work was supported by the National Natural Science Foundation of China [Grant No. 12175114 and U2167209] and Tsinghua University Initiative Scientific Research Program.
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