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The magnetic recoil spectrometer for measurements of the absolute neutron spectrum at OMEGA and the NIF
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10.1063/1.4796042
/content/aip/journal/rsi/84/4/10.1063/1.4796042
http://aip.metastore.ingenta.com/content/aip/journal/rsi/84/4/10.1063/1.4796042

Figures

Image of FIG. 1.
FIG. 1.

A schematic drawing of the MRS and its main components: a CH2 (or CD2) foil, magnet, and an array of CR-39 detectors. The MRS uses a foil positioned 10 cm away from the implosion on OMEGA (and 26 cm on the NIF) to convert incident neutrons to charged recoil particles. The measured recoil-particle spectrum is then used to determine the neutron spectrum from the implosion. Reprinted with permission from Related Article(s): J. A. Frenje et al. , Phys. Plasmas17, 056311 (Year: 2010)10.1063/1.3304475

. Copyright 2010 American Institute of Physics.

Image of FIG. 2.
FIG. 2.

(a) An image of the OMEGA MRS, surrounded by ∼20 cm thick polyethylene shielding. The shielding weighs ∼2200 lbs and surrounds the detector housing to reduce the background neutron fluence to the required level for the DSn measurement (photo taken by Eugene Kowaluk). (b) An image of the NIF MRS fully installed on the NIF-target chamber. The MRS detector array is located behind borated gunite target-chamber wall and inside ∼6000 lbs of polyethylene shielding, which greatly reduces the background neutron fluence.

Image of FIG. 3.
FIG. 3.

(a) A CAD drawing of the OMEGA MRS, which is permanently mounted to the OMEGA target chamber. The foil is inserted to a distance of 10 cm from the implosion by the nuclear diagnostic inserter (NDI). The magnet is enclosed by an aluminum vacuum housing, which is connected to the detector vacuum housing. The detector housing is surrounded by ∼2200 lbs of polyethylene shielding (shown here as a transparent material). Access to the CR-39 detector array is through the rear door. Reprinted with permission from Related Article(s): J. A. Frenje et al. , Rev. Sci. Instrum.79, 10E502 (Year: 2008)10.1063/1.2956837

. Copyright 2008 American Institute of Physics. (b) A CAD drawing of the NIF MRS positioned onto the target chamber at the line-of-sight 77°–324°. A vertical crosscut is made through the MRS to illustrate the various components in the system, i.e., the magnet, CR-39 detector array, alignment system, and shielding. The Diagnostic Insertion Manipulator (DIM) 90°–315°, not shown in this figure, is used to insert the foil to a distance of 26 cm from the implosion. The MRS is fully surrounded by ∼6000 lbs of polyethylene shielding and is positioned in the shadow of the 50 cm borated gunite cladding on top of the 10 cm thick aluminum target chamber. Reprinted with permission from Related Article(s): J. A. Frenje et al. , Phys. Plasmas17, 056311 (Year: 2010)10.1063/1.3304475

. Copyright 2010 American Institute of Physics.

Image of FIG. 4.
FIG. 4.

(a) A CAD drawing of the OMEGA MRS foil holder, illustrating the 500 μm stainless steel blast shield, the foil, which is flush against the blast shield, and the insertion rod that attaches to the NDI. (b) A CAD drawing of the NIF MRS foil holder, which illustrates the tapered arm and Ta blast shield (described in the text), the foil holder attached directly to the foil and offset from the blast shield by 5 mm, and the bracket that attaches to the DIM. (c) OMEGA MRS foil holder with the 13.2 cm2 low resolution CD2 foil attached. (d) NIF MRS foil holder with the 12.8 cm2 low-resolution CD2 foil attached (blast shield not shown).

Image of FIG. 5.
FIG. 5.

(a) MCNPX simulated fluence of 14 MeV neutrons coming directly from the implosion and neutrons scattered by the foil blast shield for the OMEGA MRS (red curve) and the NIF MRS (blue curve). The OMEGA MRS foil blast-shield is 500 μm thick and made of stainless steel and the NIF MRS foil blast-shield is 1.57 mm thick (on average) and made of tantalum. The total NIF MRS neutron fluence, per produced neutron, is lower than the OMEGA MRS because the foil is further away from the implosion. (b) MCNPX simulated fluence of recoil deuterons at the magnet aperture that originate from the neutron-fluence spectra shown in (a). A 260 μm thick CD2 foil (low resolution) was used in these simulations. As described in the text, the fluence of these recoil deuterons is insignificant when diagnosing high-ρR implosions at OMEGA and the NIF.

Image of FIG. 6.
FIG. 6.

(a) Image of the NIF MRS magnet built by Dexter Magnetic Technologies, Inc. The magnet pole gap has been plugged with polyethylene to prevent ferromagnetic objects from entering the ∼0.9 T field and possibly damaging equipment or causing injury (the plug was removed upon installation). (b) A CAD model of the MRS magnet with an image of the alignment graticules used to position and orient the magnetic field with respect to foil and CR-39 detectors. The magnets for the OMEGA MRS and NIF MRS are nominally identical but have slightly different, as built, field strengths.

Image of FIG. 7.
FIG. 7.

Modeled and measured magnetic-field maps of the main B-field component for the NIF MRS (which is nominally identical to the OMEGA MRS magnet except for a slight difference in field strength, see Figure 8 ). A comparison of the images shows a very similar overall shape and maximum field strength. The magnet modeling and field measurements were performed by Dexter Magnetic Technologies Inc.

Image of FIG. 8.
FIG. 8.

A comparison of the OMEGA MRS and NIF MRS measured magnetic field strength. This line-out was taken through the center of the MRS line-of-sight. Z = 0 cm corresponds to the location of the magnet aperture at the pole entrance side, which is at 2.25 m from TCC at OMEGA and 5.96 m from TCC at the NIF (Z = 0 is also the location of the graticule in Figure 6 ).

Image of FIG. 9.
FIG. 9.

Left: CR-39 detector array for the OMEGA MRS. The base for the OMEGA MRS detector array is a cylindrical tube with a nose cone, which latches onto an alignment pin inside the detector housing. Right: Two CR-39 detector arrays for the NIF MRS. The NIF MRS array is a flat “ironing board,” which slides along a fin to a hard stop at the end of the array. Both systems use detector “flag poles,” which position each CR-39 detector inside locking forks.

Image of FIG. 10.
FIG. 10.

The insertion of the CR-39 detector array into the OMEGA MRS (left) and NIF MRS (right). A door in the polyethylene shielding (described in detail in Sec. III E ) allows access to the MRS vacuum chamber.

Image of FIG. 11.
FIG. 11.

Position and orientation of the 11 OMEGA MRS detectors (also called windows) along the focal plane. Each CR-39 detector is designated with a sequentially increasing (with energy) window number (W1, W2, etc.), as illustrated by several examples. The trajectories of recoil particles with proton equivalent energies of 6, 10, 14, 18, and 28 MeV are also shown. Each CR-39 detector is oriented to make sure the directions of the incoming recoil particles are nearly perpendicular to the detector surface, an important feature for optimal detection with CR-39. The nine NIF MRS detectors are located and oriented in a similar way but are spaced closer together due to different ion-optical properties of the system.

Image of FIG. 12.
FIG. 12.

(a) A schematic drawing of the OMEGA MRS neutron shielding design. A stainless steel plug is used to attenuate direct unscattered DT neutrons, while polyethylene around the MRS-detector housing is used to attenuate lower energy scattered neutrons. (b) A schematic drawing of the NIF MRS neutron shielding design. The NIF target chamber (10 cm of aluminum and 50 cm of concrete) significantly moderates and attenuates direct, unscattered 14 MeV neutrons, while the polyethylene around the whole diagnostic is used to moderate and attenuate lower-energy scattered neutrons.

Image of FIG. 13.
FIG. 13.

Alignment procedure for the OMEGA MRS. Cross-hairs in the front and rear of the magnet, which define the MRS LOS, are aligned to the cross-hair in the alignment telescope that is behind the magnet. Using these cross-hairs, the MRS is pointed to TCC where a 1-mm backlit Au sphere is positioned (see the bottom left images). When the MRS is pointing towards TCC, the foil is inserted and centered on the MRS LOS at the specified distance from TCC. This alignment procedure is similar to the NIF MRS except for a minor difference, as described in the text.

Image of FIG. 14.
FIG. 14.

X-ray radiographs of CD2 foils made by GA. These images show that the CD2 foils have excellent uniformity. The difference in apparent contrasts is due to higher X-ray attenuation by the thicker foils.

Image of FIG. 15.
FIG. 15.

Images of the CD2 foils for the OMEGA MRS and NIF MRS, which illustrate the foil-holder warping and foil-cupping issues. The warping issue, which is mainly caused by intense heat exposure, only exists for the OMEGA MRS, as no protective blast shield is positioned in front of the foil holder (a 500 μm thick stainless steel plate in front of and in direct contact with the foil). The foil-cupping issue, on the other hand, exists for both the OMEGA MRS and NIF MRS. These issues result in a distance offset of about 2–5 mm from the nominal value for the OMEGA MRS and up to 3 mm for the NIF MRS. In case of the NIF MRS, this is a less significant issue as the nominal foil distance is 26 cm.

Image of FIG. 16.
FIG. 16.

Image of the OMEGA MRS low resolution CD2 foil. The measured area of the foil is 13.2 ± 0.3 cm2. To avoid interference between the foil and gate valve during the foil insertion process, the shape of the foil had to be non-circular (this is only an issue for the foils larger than 11 cm2). Also shown, is a United States nickel used as a reference area for the foil area measurement, as described in the text. (b) The measured thickness profile of the OMEGA low-resolution foil. The mean thickness is 261 ± 2 μm, and the thickness variation across the foil is characterized by a standard deviation of 18 μm.

Image of FIG. 17.
FIG. 17.

Image of the NIF MRS low resolution CD2 foil (produced and characterized by GA). The area and average thickness of this foil is 12.8 ± 0.3 cm2 and 259 ± 2 μm, respectively. The thickness variation across the foil is characterized by a standard deviation of 5 μm. Note that the blue areas are outside the foil.

Image of FIG. 18.
FIG. 18.

(a) The differential cross section for the elastic n-d scattering at neutron energies of 5.6 (dashed line) and 14.17 MeV (solid line) as a function of laboratory scattering angle. The laboratory scattering angle θr is the angle between the incoming neutron and the outgoing recoil deuteron. These cross sections were obtained from the ENDF/B-VII.0 library. 31 The location and geometry of the aperture with respect to the foil is such that only forward scattered recoil particles (θr ∼ 0) are accepted. (b) The total cross section for the elastic n-d scattering, which can be obtained by integrating the angular differential scattering cross sections, shown in (a). The total cross sections were also obtained from the ENDF/B-VII.0 library. 31

Image of FIG. 19.
FIG. 19.

(a) Image of the OMEGA MRS 11 × 2 cm2 aperture attached to the mounting plate, which attaches to the front of the magnet. (b) Image of MRS aperture super-imposed on a CAD drawing of the MRS magnet to illustrate the relative location of the aperture on the magnet and where the aperture plate attaches.

Image of FIG. 20.
FIG. 20.

Illustration of the OMEGA MRS and the MRS foil inserter (NDI) with super-imposed deuteron trajectories simulated with Geant4. The location where the NDI intercepts the path of the recoil deuterons from the foil is close to the coupling between the foil-holder arm and the NDI.

Image of FIG. 21.
FIG. 21.

(a) Measured and simulated signal distributions in the direction perpendicular to the bending plane (geometry illustrated in Figure 20 ) for the OMEGA MRS operated in medium resolution. The data was summed over shots 55 983–55 989. The dashed black curve represents a simulation where the NDI does not intercept the recoil-deuteron beam and the solid blue curve represents a simulation with the NDI intercepting the recoil-deuteron beam. Both simulations are normalized to the measure data. The simulation of the NDI interception was obtained by adjusting the height of NDI foil-holder coupling until the best fit to the data was found (a ∼2–3 mm incursion into the MRS LOS). (b) Same modeling as in (a), but for the MRS operated in low resolution compared to data obtained from a summation shots 61 415 and 61 418. Both cases indicate a detection-efficiency reduction of 13%–14% due to the NDI interception for all deuteron energies. An engineering solution to this issue is currently being implemented. This interception issue is not present at the NIF.

Image of FIG. 22.
FIG. 22.

(a) Measured and simulated primary signal distributions in the direction perpendicular to the bending plane for the OMEGA MRS operated in low resolution, which indicate a transmission of T(12.5) = 0.79 ± 0.03 at a deuteron energy of 12.5 MeV (which corresponds to a neutron energy of about 14 MeV). (b) Measured and simulated primary signal distributions in the direction perpendicular to the bending plane for the NIF MRS operated in low resolution, indicating a transmission of 100% at this energy (this is true for all energies).

Image of FIG. 23.
FIG. 23.

Transmission as a function of deuteron energy for the OMEGA MRS (solid line) and NIF MRS (dashed line). The uncertainty in the OMEGA MRS transmission function, inferred from experiments with different aperture sizes, is shown by the grey region around the solid line. Due to the ion-optical properties of the NIF MRS, the transmission is 1.0 for all deuteron energies.

Image of FIG. 24.
FIG. 24.

An illustration of a Geant4 response model of the OMEGA MRS, featuring a simulation of 6, 10, 14, 18, and 28 MeV protons.

Image of FIG. 25.
FIG. 25.

Examples of ab initio modeled response functions for the OMEGA MRS. (a) Primary DT neutron spectrum (Yn = 3 × 1013, Ti = 5 keV) used in these calculations. (b) Recoil-deuteron spectra calculated for the CD2 high, medium, and low-resolution configurations (see Table I ). As shown by these spectra, there is a tradeoff between efficiency and energy resolution.

Image of FIG. 26.
FIG. 26.

Examples of ab initio modeled NIF MRS-response functions. (a) Primary DT neutron spectrum (Yn = 1014, Ti = 5 keV) used in these calculations. (b) Recoil-deuteron spectra calculated for the high, medium, and low-resolution CD2-foil configurations (see Table I ). As shown by these spectra, there is a tradeoff between efficiency and energy resolution for the NIF MRS.

Image of FIG. 27.
FIG. 27.

(a) Measured and modeled energy (proton equivalent) as a function of detector window for the OMEGA MRS. The dashed curve was determined from the ab initio modeling of the MRS, in which a nominal magnetic field was used. The solid curve represents the in situ energy calibration of the MRS, in which the strength of the magnetic field was reduced by 1.1%. The data were obtained using the primary DT neutron spectrum and CH2 and CD2 foils. The energies below 15 MeV were probed by recoil protons from a CH2 foil, while energies above 15 MeV were probed with recoil deuterons from a CD2 foil. (b) Estimated calibration error calculated from the difference between the particle energy and the energy inferred from the calibration for the nominal field (blue triangles) and the −1.1% weaker field (red circles). The estimated energy error is improved from 430 keV to a symmetric ±160 keV (proton equivalent energy) when applying the −1.1% field correction.

Image of FIG. 28.
FIG. 28.

Calibration error of the NIF MRS when using the nominal field (blue triangles) and the +5% stronger field (red points). The solid points were determined using CD2 foil produced deuterons, while the open points were determined using D3He protons. Note the D3He split-filter configuration produces only one calibration point as the exact D3He proton energy must be determined using the unfiltered region because of electric field acceleration 34 and energy-loss uncertainty (and possible asymmetry). The estimated energy error improved from a shifted 1.6 MeV (proton equivalent energy) to ±120 keV using the +5% field correction, which corresponds to an error of ±70 keV neutron energy when using the CD2 foil.

Image of FIG. 29.
FIG. 29.

The response matrices for the OMEGA MRS operated in high (a), medium (b), and low resolution (c) (see Table I for these spectrometer configurations). As shown by these matrices, the energy broadening is more significant at lower energies. The response matrix is used to determine the neutron spectrum from the measured recoil deuteron spectrum. This is done by folding a modeled neutron spectrum with the response matrix and adjusting it until best fit to the measured spectrum is found.

Image of FIG. 30.
FIG. 30.

(a) OMEGA MRS spectra summed over a series of 15 μm CH-shell capsule implosions (blue and green spectra) and 20 μm CH-capsule implosions (red spectrum). Integrated implosions 54 465–54 471 (green), 58 165 and 58 209–58 210 (blue), and 54 472–54 474 (red) produced 1.0 × 1014, 5.0 × 1013, and 3.2 × 1013 primary neutrons, respectively. Due to different energy losses in the CD2 foils, the average energy of the deuterons is 12.0, 11.8, and 11.4 MeV for the high, medium, and low resolution foils, respectively. Note that kinematics also dictate some energy down-shift of the recoil deuterons. (b) Modeled neutron spectra that provide the best fits to the measured recoil-deuteron spectra in (a).

Image of FIG. 31.
FIG. 31.

Response matrices for the NIF MRS operated in high (a), medium (b), and low-resolution (c) modes.

Image of FIG. 32.
FIG. 32.

NIF MRS spectra obtained from three 4 μm SiO2-capsule implosions (N100923, N1001030, and N110217, which produced 4.8 × 1013, 2.3 × 1014, and 2.0 × 1014 neutrons, respectively). The green, blue, and red spectra were obtained when the MRS was operated in high-resolution, medium-resolution, and low-resolution mode, respectively. Due to different energy losses in the CD2 foil, the average energy of the deuterons is 11.4 MeV for low resolution, 12.0 MeV for medium resolution, and 12.3 MeV for high resolution. (b) Modeled neutron spectra that provide the best fits to the measured recoil-deuteron spectra in (a).

Image of FIG. 33.
FIG. 33.

Measured and simulated neutron fluence per produced neutron as a function of distance from the implosion (or TCC). Data and simulations are contrasted to the 1/4πR2 scaling (black curve), which illustrates the effect of scattered neutrons.

Image of FIG. 34.
FIG. 34.

(a) MCNP model of the OMEGA MRS, which includes the magnet, detector array, 20 cm of polyethylene shielding that surrounds the detector array, and a 20 cm thick piece of stainless steel positioned in front of the detector array to attenuate direct, unscattered primary neutrons via inelastic collisions. (b) Simulated map of neutron fluence per produced neutron in the region around and inside the MRS shielding. The implosion is located at the origin and the MRS LOS is oriented along the x-axis.

Image of FIG. 35.
FIG. 35.

Measured and simulated neutron fluence (per produced primary-neutron) along the OMEGA MRS detector array with and without shielding around the MRS.

Image of FIG. 36.
FIG. 36.

(a) A detailed MCNP model of the NIF target bay, which includes the NIF target chamber with its laser and diagnostic ports, and the concrete and stainless steel reinforced walls and floors. This model was initially used to determine the neutron fluence at the MRS location. The NIF MRS was subsequently added to the model. (b) Simulated neutron fluence inside the NIF target bay. The fluence is given in neutrons/cm2 per produced neutron. A neutron fluence of ∼10−7 n/cm2 per produced neutron was determined at the MRS location.

Image of FIG. 37.
FIG. 37.

(a) MCNP model of the final NIF MRS shielding design. (b) Simulated neutron fluence (Log10) around and inside the MRS shielding. The neutron fluence is given in neutrons/cm2 per produced neutron. This simulation shows that the fluence is reduced inside the shielding by about a factor of 50 to ∼2 × 10−9 n/cm2 per produced neutron.

Image of FIG. 38.
FIG. 38.

MRS measured recoil deuteron spectra for two cryogenic DT implosions (shot 55 723 black circles and 54 926 grey squares) on OMEGA (part a) and two cryogenic DT implosions (shot N110608 red circles and N110212 blue triangles) at the NIF (part b). In these experiments, the MRS was operated in low-resolution mode. These spectra are normalized by the primary neutron yield YDT to directly show the different levels of DSn (and therefore differences in ρR) at deuteron energies <10 MeV corresponding to neutron energies 10–12 MeV (see Figures 29 and 31 ). The MRS continues to regularly diagnose cryogenic DT implosions at both OMEGA and the NIF. Parts (a) and (b) reprinted with permission from Related Article(s): J. A. Frenje et al. , Phys. Plasmas17, 056311 (Year: 2010)10.1063/1.3304475

and Related Article(s): S. H. Glenzer et al. , Phys. Plasmas19, 056318 (Year: 2012)10.1063/1.4719686

, respectively. Copyright 2010 and 2012 American Institute of Physics.

Tables

Generic image for table
Table I.

System parameters for the MRS on OMEGA and the NIF and associated errors. 5 The efficiency and energy resolution for each MRS configuration are also shown.

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2014-04-18
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: The magnetic recoil spectrometer for measurements of the absolute neutron spectrum at OMEGA and the NIF
http://aip.metastore.ingenta.com/content/aip/journal/rsi/84/4/10.1063/1.4796042
10.1063/1.4796042
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