TY - JOUR
AU - Woods, M.
AB - Abstract A systematic study of photon and neutron radiation doses generated in high-intensity laser–solid interactions is underway at SLAC National Accelerator Laboratory. These laser–solid experiments are being performed using a 25 TW (up to 1 J in 40 fs) femtosecond pulsed Ti:sapphire laser at the Linac Coherent Light Source's (LCLS) Matter in Extreme Conditions (MEC) facility. Radiation measurements were performed with passive and active detectors deployed at various locations inside and outside the target chamber. Results from radiation dose measurements for laser–solid experiments at SLAC MEC in 2014 with peak intensity between 1018 and 7.1 × 1019 W cm−2 are presented. INTRODUCTION In recent years, the number and use of high-power multi-terawatt and petawatt lasers to explore laser–matter interaction in research facilities have rapidly increased around the world. They are used to study matter under extreme conditions(1), to produce energetic beams of protons and/or electrons(2, 3), or to generate forward-directed betatron X-rays(4, 5). This article focusses on four radiation measurements performed during high-intensity laser shots on solid foils at SLAC National Accelerator Laboratory's Matter in Extreme Conditions (MEC) facility in 2014. The interaction of a high-intensity laser with a solid creates a thin plasma layer on the surface of the target and accelerates the electrons in the plasma to tens of MeV in energy(6, 7). These ‘hot’ electrons will interact with the laser target and the target chamber and generate bremsstrahlung(8, 9). This mixed field of electrons and photons is a source of ionising radiation and can create a radiation hazard for personnel unless sufficient radiological controls are implemented. A variety of active and passive detectors were deployed to measure the radiation doses generated from laser–solid interactions for 25 TW laser commissioning experiments at MEC in 2014. MEC 25 TW laser For experiments in 2014, SLAC MEC utilised a Ti:sapphire short-pulse laser system with a wavelength of 0.8 µm, 1 J pulse energy and 40 fs pulse width (FWHM). This provided a laser beam with a peak power of 25 TW to deliver intense light pulses onto solid targets. An off-axis parabolic (OAP) mirror focussed the MEC laser beam to micrometre spot sizes to achieve laser intensities between 1016 and 1020 W cm−2. A target rastering system ensured each laser shot interacts with fresh target material at a laser repetition rate of 1 Hz. Laser scientists performed a characterisation of the high-intensity laser's beam parameters for each laser–solid experiment at MEC. The pulse energy was measured both before and after the compressor with a Coherent J50 50M-IR sensor and a Coherent LabMax-TOP meter (Coherent, Inc., 5100 Patrick Henry Drive, Santa Clara, CA 95054, USA). The pulse duration was measured twice with two separate instruments, a Coherent single-shot autocorrelator (SSA) and an APE LX Spider autocorrelator (APE Angewandte Physik und Elektronik GmbH, Plauener Strasse 163–165 Haus N, 1305 Berlin, Germany), before and after each experiment. A charge-coupled device (CCD) camera, such as the Admiec OPAL-1000 (Adimec Electronic Imaging, Inc., 245 North Street, Stoneham, MA 02180, USA), imaged the laser beam and determined the spot size. Figure 1 shows an image of a typical laser beam with a Gaussian-like profile achieved during laser–solid experiments at MEC. Figure 1. Open in new tabDownload slide Laser beam intensity profile from the September 2014 laser–solid experiment at MEC, 7.1×1019cm−2 ⁠. Table 1 provides the laser beam parameters from the four laser–solid experiments at MEC in 2014. MEC achieved laser intensities up to 7.1 × 1019 W cm−2. The uncertainty in achieved intensity is calculated to be ∼38 % for the February experiment and 20 % for the July–August experiments. The hot electron temperature Th is derived from laser parameters and characterises the energy of the hot electrons generated from laser–solid interactions. Further details on Th and the radiation dose yield generated from hot electrons will be described in the next section. Table 1. MEC laser beam parameters from laser–solid experiments in 2014. Experiment . Pulse energy (J) . Pulse width (fs) . Fraction of energy in peak . Peak power (TW) . 1/e2 spot size (µm × µm) . Peak intensity (W cm−2) . Th (MeV) . February 2014 1.0 70 0.19 2.8 13 × 8 1.8 × 1018 0.18 July 2014 0.7 50 0.77 10.7 37 × 19 1.0 × 1018 0.11 August 2014 0.7 50 0.44 6.1 9 × 5 1.0 × 1019 0.71 September 2014 0.5 50 0.63 6.3 3 × 2 7.1 × 1019 2.5 Experiment . Pulse energy (J) . Pulse width (fs) . Fraction of energy in peak . Peak power (TW) . 1/e2 spot size (µm × µm) . Peak intensity (W cm−2) . Th (MeV) . February 2014 1.0 70 0.19 2.8 13 × 8 1.8 × 1018 0.18 July 2014 0.7 50 0.77 10.7 37 × 19 1.0 × 1018 0.11 August 2014 0.7 50 0.44 6.1 9 × 5 1.0 × 1019 0.71 September 2014 0.5 50 0.63 6.3 3 × 2 7.1 × 1019 2.5 Open in new tab Table 1. MEC laser beam parameters from laser–solid experiments in 2014. Experiment . Pulse energy (J) . Pulse width (fs) . Fraction of energy in peak . Peak power (TW) . 1/e2 spot size (µm × µm) . Peak intensity (W cm−2) . Th (MeV) . February 2014 1.0 70 0.19 2.8 13 × 8 1.8 × 1018 0.18 July 2014 0.7 50 0.77 10.7 37 × 19 1.0 × 1018 0.11 August 2014 0.7 50 0.44 6.1 9 × 5 1.0 × 1019 0.71 September 2014 0.5 50 0.63 6.3 3 × 2 7.1 × 1019 2.5 Experiment . Pulse energy (J) . Pulse width (fs) . Fraction of energy in peak . Peak power (TW) . 1/e2 spot size (µm × µm) . Peak intensity (W cm−2) . Th (MeV) . February 2014 1.0 70 0.19 2.8 13 × 8 1.8 × 1018 0.18 July 2014 0.7 50 0.77 10.7 37 × 19 1.0 × 1018 0.11 August 2014 0.7 50 0.44 6.1 9 × 5 1.0 × 1019 0.71 September 2014 0.5 50 0.63 6.3 3 × 2 7.1 × 1019 2.5 Open in new tab SLAC RADIATION PROTECTION MODEL The temperature Th (or energy) of the hot electrons generated from laser–solid interactions is a crucial parameter to estimate the bremsstrahlung photon dose yield, and Th is calculated from laser beam parameters. The value of Th also determines how ‘hard’ the hot electron energy and bremsstrahlung spectra are, which impacts the laser-generated ionising radiation hazard. Hot electron temperature and energy distribution At lower laser intensities, inverse bremsstrahlung and resonance absorption are the dominant mechanisms for producing hot electrons. Meyerhofer et al. provides the scaling in the following equation to calculate the hot electron temperature Th in megaelectron volt from the normalised laser intensity Iλ2(10): Th=6×10−8(Iλ2)0.33.(1) At higher laser intensities, the ponderomotive force is the primary electron heating mechanism, and it is defined as the force that a dipole experiences in an oscillating electromagnetic field. For laser–plasma interaction, the electrons in the plasma experience the oscillating electric field of the incident laser pulse and gain energy. Equation 2 calculates the hot electron temperature Th based on the ponderomotive force for Iλ2>1.6×1017Wμm2cm−2. The mec2 term is the electron rest mass of 0.511 MeV(11–13): Th=mec2−1.0+1.0+Iλ21.37×1018.(2) It is straightforward to calculate Th from the laser beam parameters provided in Table 1 and with λ=0.8μm. The value of Th can easily reach the megaelectron volt energy range as laser intensity increases. The values of Th for the experiments in 2014 are given in Table 1 and range from 0.18 MeV at 1.8×1018Wcm−2 up to 2.5 MeV at 7.1×1019Wcm−2 ⁠. Depending on the laser intensity, the distribution of the hot electrons generated from laser–solid interactions is often described as either a Maxwellian or relativistic Maxwellian distribution(14, 15). The following equations provide the two distributions used by SLAC in characterising the hot electron spectra: Ne∝E1/2e−E/ThforI≤1019Wcm−2,(3) Ne∝E2e−E/ThforI>1019W cm−2.(4) For I>1019Wcm−2, the average energy of the relativistic Maxwellian spectrum is 3Th, and electron energies in the tail portion of the hot electron spectrum can easily reach tens of megaelectron volts. The bremsstrahlung photons generated from MeV electrons can pose a challenge for a high-power laser facility unless proper radiation shielding and controls are in place to mitigate the dose to personnel in the vicinity. Dose equivalent from bremsstrahlung photons Estimation of the bremsstrahlung dose (from hot electrons) is crucial in performing dose mitigation and establishing controls for high-power laser experiments. Monte Carlo codes such as FLUKA can calculate the dose equivalent from a known hot electron source term with a distribution described in Equations 3 and 4 and characterised by Th in Equation 2. However, a simple empirical formula based on laser parameters and Th can provide a quick estimate: Hx≈1.8×1.10×αR2Th2forTh<3MeV,(5) Hx≈1.8×3.32×αR2ThforTh≥3MeV.(6) Hayashi et al. established a dose yield model for estimating the 0° forward dose equivalent generated from a high-power laser interacting with a thick solid target for Iλ2=1019−1021Wμm2cm−2 or about Th=1−13MeV(16). Equations 5 and 6 show the dose yield formulas proposed by Hayashi et al., where α is the laser energy to electron energy conversion efficiency, and R is the distance from the laser–solid interaction point in centimetres. The dose yield parameter Hx is the dose equivalent (Sv) generated per joule (J) of laser energy in the 0° laser forward direction. The SLAC radiation protection (RP) model expanded the dose model by Hayashi et al. to include laser intensities down to 1016Wcm−2 with Th calculated from Equations 1 and 2(17), and with the laser conversion efficiency α taken from work by Key et al. to be 30 % for I≤1019Wcm−2 and 50 % for I>1019Wcm−2(18). Figure 2 shows the SLAC RP 0° dose yield model at 1 m as a function of intensity with λ=0.8μm. Also in Figure 2 are the maximum dose yields from several laser–solid experiments at SLAC's MEC and also one at Lawrence Livermore National Laboratory's (LLNL) Titan. The data points are collected from measurements at angles around the target chamber, not exclusively in the 0° direction. Figure 2. Open in new tabDownload slide SLAC RP models for dose yield at 1 m plotted with measured photon dose yield data from laser–solid experiments (λ=0.8μm). The types of targets used during each laser–solid experiment are indicated in the legend. SLAC's adjusted RP model is scaled down to better reflect measurement data (see text for description) and also includes an attenuation factor for 2.54-cm Al. Not all the dose yield points were measured in the 0° direction. The SLAC RP model overestimates the dose yield by when compared with the measurement data for laser intensities at 1018Wcm−2 and above. Therefore, the dose yield model was scaled down by a factor of 1/10 for laser intensities ≥1019Wcm−2 and ramped down continuously from 1019 to 1018Wcm−2 to better fit the measurement data. Attenuation factors for 2.54 cm Al are also applied to the adjusted model to take into account the shielding effect of the target chamber wall. This adjusted model is not a perfect fit to the measurement data, but it provides a conservative and more realistic estimate of the radiation hazard generated from laser–solid experiments than the Hayashi dose model. SLAC currently uses the adjusted model to determine which radiological controls or shielding is needed to mitigate the dose to personnel. MEC RADIATION DOSE MEASUREMENTS A combination of passive and active detectors inside and around the outside of the MEC target chamber measured the radiation dose and dose equivalent generated from laser–solid interactions for different laser intensities. The target chamber is composed of aluminium, has a wall thickness between 2 and 6.4 cm, a radius of ∼1 m and is pumped down to vacuum during high-intensity laser shots on target. Passive detectors measured the integrated dose (mGy) or dose equivalent (µSv) over a series of many laser shots on target, while active detectors provided dose equivalent rate (µSv h−1) measurements in real time during laser shots on target. Spectrometers were also used in an attempt to characterise the hot electron energy spectra. The results from the spectrometer measurements are not detailed here and will be discussed in a future publication. Table 2 provides a comprehensive list of the solid foils and their thicknesses that were used at targets during the laser–solid experiments at MEC in 2014. Table 2. MEC laser shots and target description. Intensity (W cm−2) . Target material . Thickness (µm) . Number of laser shots . 1.8×1018 Cu 100 540 1.0×1018 Cu + Kapton 5 + 30 550 Ni 15 275 Cu 100 655 1.0×1019 Cu 100 340 Ni 15 220 7.1×1019 Al 15 and 10 70 and 66 Au 5 22 Cu 5 26 CH3 4 and 2.5 6 and 37 Intensity (W cm−2) . Target material . Thickness (µm) . Number of laser shots . 1.8×1018 Cu 100 540 1.0×1018 Cu + Kapton 5 + 30 550 Ni 15 275 Cu 100 655 1.0×1019 Cu 100 340 Ni 15 220 7.1×1019 Al 15 and 10 70 and 66 Au 5 22 Cu 5 26 CH3 4 and 2.5 6 and 37 Open in new tab Table 2. MEC laser shots and target description. Intensity (W cm−2) . Target material . Thickness (µm) . Number of laser shots . 1.8×1018 Cu 100 540 1.0×1018 Cu + Kapton 5 + 30 550 Ni 15 275 Cu 100 655 1.0×1019 Cu 100 340 Ni 15 220 7.1×1019 Al 15 and 10 70 and 66 Au 5 22 Cu 5 26 CH3 4 and 2.5 6 and 37 Intensity (W cm−2) . Target material . Thickness (µm) . Number of laser shots . 1.8×1018 Cu 100 540 1.0×1018 Cu + Kapton 5 + 30 550 Ni 15 275 Cu 100 655 1.0×1019 Cu 100 340 Ni 15 220 7.1×1019 Al 15 and 10 70 and 66 Au 5 22 Cu 5 26 CH3 4 and 2.5 6 and 37 Open in new tab Dose inside target chamber Small 1 cm × 1 cm passive (non-electronic) nanoDot (LANDAUER, 2 Science Road, Glenwood, IL 60425, USA) dosimeters from Landauer were used inside the MEC target chamber during laser–solid experiments for different target types and laser intensities. The nanoDot dosimeters were deployed at 30 cm distances radially around the laser–target interaction point and measured the dose in mGy from a mixed field of hot electrons and bremsstrahlung photons. Figure 3 shows the dose measured by nanoDots for laser intensities 1.8×1018 and 1018Wcm−2 ⁠. The laser–target interaction point is at the centre of the radial plot, and the laser axis is indicated on each plot. The dose has been normalised to the number of laser shots delivered onto the solid target during each run. The dose profiles indicate that the mixed electron and photon fields generated from laser–solid interactions are emitted primarily in the forward and backward laser axis directions. Figure 3. Open in new tabDownload slide Dose per shot at 30 cm inside the target chamber for 1018Wcm−2. The nanoDot at ∼225° was blocked before Run 3 (100-µm Cu) by an Al shield that was inserted to protect the OAP mirror. The doses measured in the backward laser direction agree well with a maximum of ∼15 mGy/shot. In contrast, the doses measured in the forward laser axis direction suggest a dependence on target thickness. The nanoDots in the forward direction measured less dose during shots on 100-µm copper than during shots on the thinner 5-µm copper and 15-µm nickel. Because the hot electron temperature is ∼100–200 keV at 1018Wcm−2 (from Equation 2), the self-shielding effect of the thicker 100-µm copper attenuates a large fraction of the hot electrons emitted in the forward direction. Figures 4 and 5 show the radial dose profiles for two other laser–solid experiments at MEC for laser intensities of 1019 and 7.1×1019Wcm−2. Looking at Figures 3–5, an increase in laser intensity leads to an increase in dose per shot generated inside the target chamber, as expected. Furthermore, the dose profiles become more forward peaked with increasing laser intensity. At 1019Wcm−2, the dose is slightly forward peaked up to around 12 mGy/shot, and then at 7.1×1019Wcm−2, the dose per shot is sharply forward peaked up to 45 mGy/shot. The shape and magnitude of the dose profiles depend on the laser intensity and the target thickness. Figure 4. Open in new tabDownload slide Dose per shot at 30 cm inside the target chamber for 1019Wcm−2. A nanoDot was deployed outside the target chamber at a very thin diamond view port during each run, and the dose per shot was normalised to a distance of 30 cm. Figure 5. Open in new tabDownload slide Dose per shot at 30 cm inside the target chamber for 7.1×1019Wcm−2 ⁠. Photon dose outside target chamber Hot electrons generated from laser–solid interactions will interact with the target material and the chamber walls to generate a bremsstrahlung photon field. Victoreen 451P (Fluke Biomedical, 6045 Cochran Road, Cleveland, OH 44139, USA) ion chambers were positioned at different angles around the outside of the MEC target chamber and recorded the ambient photon dose equivalent, H*(10), rate as microsieverts per hour in real time. For the remainder of this discussion, ambient dose equivalent rate will be referred to as simply dose rate. The aluminium MEC target chamber is shaped like an octagon with a radius of ∼1 m. The octagonal target chamber has eight access doors along the ‘sides’ and eight flanges (location of view ports) at the ‘corners’. The chamber's doors are ∼6.4-cm-thick Al, and the flanges are ∼2-cm-thick Al. The ion chambers were deployed in the forward and backward laser axis directions and at view ports if they were available. Ion chambers were also deployed at increasing radial distances to observe the drop in dose rate over distance. Figure 6 presents an example of the photon dose rates measured by a Victoreen 451P ion chamber from the July 2014 laser–matter experiment at MEC. The ion chamber's dose rates over time agree well between ∼20 and 40 µSv h−1 for three different solid targets and also demonstrate good shot by shot stability of the laser intensity while operating at 1 Hz repetition rate. Figures 7–9 provide the maximum photon dose rates measured at MEC for laser shots on solid targets from 1018 to 7.1×1020Wcm−2. The dose rates have not been normalised and are shown ‘as measured’. Figure 6. Open in new tabDownload slide Photon dose rates measured by Victoreen 451P ion chamber at r = 1.3 m and θ=+23° at MEC during July 2014 for 1018Wcm−2 ⁠. Figure 7. Open in new tabDownload slide Maximum photon dose rates during each run at MEC during July 2014 for 1018Wcm−2 ⁠. Figure 8. Open in new tabDownload slide Maximum photon dose rates during each run at MEC during August 2014 for 1019Wcm−2 ⁠. Figure 9. Open in new tabDownload slide Maximum photon dose rates during each run at MEC during September 2014 for 7.1×1019W cm−2 ⁠. July 2014 at MEC, 1018 W cm−2 In Figure 7 for 1018 W cm−2, the photon dose rates measured at the two view ports (or flanges) at 23° in the forward and backward laser axis directions agree well, and the same is evident at the chamber's doors at 0° and 45°. The photon dose rates of 30–50 µSv h−1 at the chamber's flanges are consistently higher by about a factor of 10 than the 4–5 µSv h−1 at the chamber doors. As a reminder, the aluminium target chamber doors are 6.4 cm thick, and the flanges are 2 cm thick. The Victoreen 451P ion chambers located at the doors measured less photon dose than the ones at the flanges due to aluminium attenuation. The two ion chambers located in the 0° forward direction measured ∼5 and 1 µSv h−1 at 1.4 and 3.2 m distances from the laser–target interaction point, respectively, and the dose rate at 3.2 m is lower than at 1.4 m by a factor of 1/5. This behaviour at 1018Wcm−2 operation suggests the photon dose falls off as 1/r2 and originates from the centre of the target chamber, whereas hot electrons interact with the solid target and generate bremsstrahlung. Dependence on material type (copper or nickel) and target thickness has negligible effect on the measured bremsstrahlung dose rates outside the target chamber for 1018Wcm−2 laser–solid experiments, and the photon dose rates at every location were within about a factor of 2 between runs. August 2014 at MEC, 1019Wcm−2 Figure 8 shows the maximum photon dose rates measured by ion chambers at MEC for two runs during a 1019Wcm−2 laser–solid experiment. As expected, the photon dose rates for 1019Wcm−2 are higher than for 1018Wcm−2 (earlier in Figure 7) and do not scale linearly with laser intensity. At 1019Wcm−2, the photon dose rates generated from laser shots on 100-µm Cu are consistently higher within about a factor of 2 at all locations than from shots on 15-µm Ni. Since the Cu (Z = 29) and Ni (Z = 28) targets have similar mass densities, the higher photon dose rate measured for Cu may be because the Cu target is a little more than six times thicker than the Ni target, such that hot electrons produced from laser–solid interactions simply interact with more material and generate more bremsstrahlung in the 100-µm Cu target than in the 15-µm Ni target. The photon dose rates are similar at the flanges at 23° in the forward and backward directions, and they are also consistently higher than the dose rates at the chamber's doors at 0° and 45°. Again, the difference in aluminium thickness between the flanges (2 cm) and the doors (6.4 cm) account for the difference in photon dose rates due to attenuation. In the 0° laser forward direction, the photon dose rates for both runs fall off with distance as 1/r2 ⁠. Also indicated in Figure 8 is a measurement made by a nanoDot dosimeter outside a small diamond view port with direct line of sight to the laser–target interaction location. The diamond view port was 100 µm thick with a radius of 1 cm and was located at 90° from the laser axis. Since the total number of shots and the laser repetition rate are known parameters, the integrated dose measured by the passive nanoDot dosimeter can be converted into a dose rate. The diamond view port dose rates are at least three orders of magnitude greater than what was measured around the aluminium chamber. Because 100 µm of diamond provides little to no shielding, measurements at the diamond view port represent the dose rate (from electrons and photons) at 1 m if no shielding is present. The diamond view port's nanoDot measurement can also be normalised to the total laser shots in a run and to a distance of 30 cm to obtain dose per shot inside the target chamber. The now normalised dose per shot at 30 cm agrees well within a factor of 2 to the nanoDot measurements inside the target chamber that were provided earlier in Figure 4. September 2014 at MEC, 7.1×1019W cm−2 Radiation dose measurements in September 2014 at MEC were performed concurrent with another high-power laser experiment at 7.1×1019Wcm−2. To mitigate the radiation hazard to personnel, two 2.54-cm-thick tungsten alloy (70 and 93 %) shields were deployed in the forward and backward laser axis directions. Victoreen 451P ion chambers were positioned around the target chamber and on the roof, but the tungsten shielding blocked the ion chamber in the forward direction of the laser at 6°. The shielding did not affect the other ion chambers. Figure 9 shows the maximum photon dose rates measured by the ion chambers during runs 1–4 at MEC for 7.1×1019Wcm−2 ⁠. The MEC laser delivered continuous shots at 1 Hz onto the solid aluminium targets during Runs 1 and 3. The ion chambers at 90° and 68° measured very high photon dose rates of 2060 and 2740 µSv h−1 during Run 1, and dose rates of 4390 and 3910 µSv h−1 during Run 3. The ion chamber located at 6° in the laser forward direction measured 585 and 116 µSv h−1, even though it was shielded by the 2.54-cm tungsten. Runs 2 and 4 during the September 2014 laser–solid experiment did not utilise the MEC laser's continuous 1 Hz repetition rate. Instead, the laser system delivered single laser shots (frequency separated by up to 1 or more minutes) onto the solid targets. The ion chambers did not respond well for shot-by-shot detection, and their dose rate readings under-responded during Runs 2 and 4 compared with Runs 1 and 3. For example, the ion chambers at 90° and 68° measured 16 and 33 µSv h−1 during Run 2 and 54 and 14 µSv h−1 during Run 4, while they measured in the thousands of microsieverts per hour during Runs 1 and 3. The ion chambers also measured about an order of magnitude less dose rate during Runs 2 and 4 in the laser forward direction at 6° even with the 2.54-cm tungsten alloy shielding. The ion chamber deployed on the roof of the chamber measured a maximum dose rate of 610 µSv h−1, which occurred during continuous laser shots on 10-µm Al. Unlike the other locations around the chamber, the roof did not measure significantly less dose during Runs 2 and 4. Photon dose measured by passive detectors at MEC RADOS RAD-60 [Mirion Technologies (MGP), Inc., 5000 Highlands Parkway, Suite 150, Smyrna, GA 30082, USA) electronic dosimeters and Arrow-Tech Model 2 (Arrow-Tech, Inc., 417 Main Ave W, Rolla, ND 58367, USA) (range of 0–20 µSv) pocket ion chambers or PICs were also deployed around the outside of the target chamber during laser–solid experiments at MEC. These passive instruments measured the integrated ambient dose equivalent from all shots onto the solid target during a run. The integrated ambient doses were normalised to the total number of laser shots on target and the laser repetition rate and compared with measurements made by the active Victoreen 451P ion chambers. Measurements from passive dosimeters and active ion chambers were found to be in good agreement (especially between the 0 and 20 µSv PICs and the ion chambers) when enough laser shots were taken on target to generate an accumulated dose. The RAD-60 electronic dosimeters began to under-respond compared with the PICs at laser intensities > 1019Wcm−2 ⁠. Neutron dose at MEC Polyethylene-moderated BF3 tubes (designed in-house at SLAC) were deployed around the target chamber and measured the neutron fluence generated from laser–solid experiments at MEC. Neutrons are generated primarily from photonuclear (γ,n) interactions when high-energy bremsstrahlung from hot electrons interacts with the target material or the chamber walls. The BF3 detectors are calibrated with an 11-GBq PuBe neutron source and compared with the ambient neutron dose equivalent rate measured by a Model 5085 Meridian (Health Physics Instruments, 330 D South Kellogg Ave., Goleta, CA 93117, USA) neutron survey meter. A conversion factor is derived for each BF3 detector to convert the neutron fluence to ambient dose equivalent. A typical conversion factor for a BF3 is ∼105 counts per µSv. Figure 10 shows an example of the measurements performed by both a BF3 and a Victoreen 451P ion chamber. Whenever the ion chambers measured photons, the BF3 also measured neutrons at the same time. In addition, the instruments showed that a neutron dose rate of ∼3 to 4×10−2μSv h−1 was consistently generated from a photon dose rate of ∼1 µSv h−1. Figure 10. Open in new tabDownload slide Photon and neutron dose rates at MEC during July 2014 for 1018Wcm−2 ⁠. Please note the different scales for photons and neutrons. The ambient neutron dose equivalent rates (µSv h−1) can be normalised into a neutron dose yield (µSv J−1) since laser beam parameters such as laser repetition rate and pulse energy are well characterised. Figure 11 shows the neutron dose yields normalised to a distance of 1 m from laser–solid interactions and also indicates the distance from the target chamber centre and angle relative to the laser direction. The data suggest that the neutron dose yield increases with laser intensity, which is expected. The prompt neutron dose rate is small compared with the prompt photon dose rates; but at higher intensities, it has the potential to activate equipment inside and around the target chamber. No correlation between the neutron yields and the locations around the target chamber is evident. Figure 11. Open in new tabDownload slide Neutron dose yield at 1 m from laser–solid experiments at MEC. The measurement locations are indicated next to each data point as the distance from the target chamber centre and the angle relative to the laser direction. DISCUSSION As presented earlier in Figure 2, radiation measurements by SLAC have covered a wide range of laser intensities from 1016to1021Wcm−2. The data points in the figure represent the maximum dose yields at 1 m measured by a combination of active and passive detectors during each experiment. Studies by Bauer et al. have detailed elsewhere the radiation dose measurements performed at LLNL's Titan laser facility in 2011 (between 1020and1021Wcm−2 ⁠) and at SLAC's MEC in 2012 (between 1016and1018Wcm−2 ⁠). The dose yields from these two laser–solid experiments agree well with the trend of increasing dose yield as a function of laser intensity, and the measurements below 1018Wcm−2 agree especially well with the RP models. Bauer et al. also observed at Titan that electronic-based detectors around the target chamber (i.e. ion chambers and RAD-60) did not respond during laser shots on solid target, and only passive PICs responded properly(19, 20). The dose yields from laser–solid experiments in 2014 agree well within a factor of 2–3 when compared with the adjusted model(21). The differences between the measurement data and the dose yield models may be due to uncertainties behind the derivation of the model. The model's laser-to-electron conversion efficiency α of 30–50 % may be too optimistic. A single parameter, such as the hot electron temperature Th, may not be enough to fully characterise the time-dependent fluctuating temperature of the laser-induced plasma. The transition of the hot electron spectrum from a Maxwellian distribution to a ‘hotter’ relativistic Maxwellian at 1019Wcm−2 (Equations 3 and 4) may actually occur at a higher laser intensity. For example, a shift in the hot electron spectrum from a relativistic Maxwellian distribution to just Maxwellian would significantly affect the attenuation factor by 2.54-cm Al that is applied to the dose yield model. If the transition to a relativistic Maxwellian distribution occurs at 1020Wcm−2, the dose yield model at 1019Wcm−2 will be lower because attenuation by 2.54-cm Al at 1019Wcm−2 will be higher. The measurements at 7.1×1019W cm−2 are about a factor of 1/3 below the adjusted model. It should be noted that tungsten local shielding blocked the instruments in the forward laser direction during this experiment, so the dose yields are from other angles. Since the RP model estimates the dose yield in the 0° forward direction, it is expected that the detectors at other angles will measure less than the model due to the very forward-directed nature of the dose yield at high laser intensities. SUMMARY The RP department at SLAC and colleagues at MEC have measured ionising radiation generated from the interaction of a high-intensity laser with solid targets between 1016and1021W cm−2. The laser beam parameters are well characterised, and sustained laser shots on different solid targets were delivered. A combination of passive and active detectors were used to measure the dose yield in millisievert per joule outside the target chamber, and the SLAC RP dose yield model was adjusted to better reflect the measurement data. Inside the target chamber, passive nanoDot dosimeters measured very high doses per shot up to 45 mGy per shot in the forward direction of the laser at 7.1×1019W cm−2, and showed that the radial dose levels increase and become more forward peaked with increasing laser intensity. Outside the target chamber, active Victoreen 451P ion chambers and passive RAD-60 dosimeters and 0–20 µSv PICs characterised the ambient photon dose equivalent at various angles and recorded the data in real time. BF3 detectors also measured ambient neutron dose equivalent rates that coincided with ion chambers' measurements. SLAC's RP programme utilises the adjusted dose yield model to design radiation shielding and establish controls at the MEC laser facility in order to mitigate the radiation hazard to its personnel. FUNDING This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-76SF00515. ACKNOWLEDGEMENTS The author wishes to thank the staff of SLAC from RP and Linac Coherent Light Source and also N. Hertel of Georgia Institute of Technology. REFERENCES 1 Fletcher L. B. et al. . Ultrabright X-ray laser scattering for dynamic warm dense matter physics . Nat. 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Measurements of Ionizing Radiation Doses Induced by High Irradiance Laser on Targets in LCLS MEC Instrument . SLAC Publication SLAC PUB–15889 , 1 – 39 ( 2013 ). 21 Liang T. et al. . Measurements of High-Intensity Laser Induced Ionising Radiation at SLAC . Shielding Aspects of Accelerators, Targets and Irradiation Facilities SATIF-12 Workshop Proceedings , 40 – 53 ( 2015 ). Published by Oxford University Press 2015. This work is written by (a) US Government employee(s) and is in the public domain in the US. Published by Oxford University Press 2015. This work is written by (a) US Government employee(s) and is in the public domain in the US.
TI - RADIATION DOSE MEASUREMENTS FOR HIGH-INTENSITY LASER INTERACTIONS WITH SOLID TARGETS AT SLAC
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncv505
DA - 2016-12-01
UR - https://www.deepdyve.com/lp/oxford-university-press/radiation-dose-measurements-for-high-intensity-laser-interactions-with-au8xqQ8bwc
SP - 346
EP - 355
VL - 172
IS - 4
DP - DeepDyve
ER -