PFC/JA MIT Fusion 7-Ray Diagnostic Development. K. W. Wenzel, R. D. Petrasso, D. H. Lo, C. K. Li,

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1 PFC/JA-92-5 MIT Fusion 7-Ray Diagnostic Development K. W. Wenzel, R. D. Petrasso, D. H. Lo, C. K. Li, J. W. Coleman, J. R. Lierzer, E. Hsieht and T. Bernatt Submitted for publication in: Review of Scientific Inatruments Plasma Fusion Center Massachusetts Institute of Technology Cambridge, MA f Lawrence Livermore National Laboratory Livermore, CA This work was supported in part by U.S. DOE Grant No. DE-FG02-91ER54109 and LLNL Subcontract No. B

2 MIT Fusion y-ray Diagnostic Development K. W. Wenzelt, R. D. Petrasso, D. H. Lot, C. K. Li, J. W. Coleman, and J. R. Lierzer, MIT Plasma Fusion Center, Cambridge MA E. Hsieh and T. Bernat Lawrence Livermore National Laboratory, Livermore CA March 1992 Abstract In conjunction with -y-ray diagnostic development for the NOVA Upgrade and Alcator C-MOD experiments, we have assembled a comprehensive set of absolutely-calibrated -y-ray sources and several y-ray detectors. These tools will be used in characterizing and calibrating new y-ray diagnostic systems as they are assembled. The 7-ray sources comprise both radioisotopes and nuclear reactions. The latter are generated in the MIT Cockcroft-Walton fusion-product generator. The detectors include several "standard" NaI(Tl) scintillators and a high-resolution germanium detector. This paper briefly discusses the diagnostics planned for NOVA Upgrade and Alcator C-MOD. In addition it describes the 7-ray sources and detector characterizations we have performed in the laboratory. 1

3 I. Introduction -ray diagnostics have certain advantages over other fusion-product diagnostics, because -y rays escape the fusion plasma and its vacuum vessel without suffering significant scattering. Charged particles, by contrast, are largely confined in the plasma, and neutrons are scattered by the surrounding structure. Thus the -y-ray branches of fusion reactions provide a unique opportunity to measure the fusion yield and the fusion burn history. We are developing 7-ray diagnostics for the NOVA Upgrade ICF experiment and the Alcator C-MOD tokamak experiment. The primary objectives are measurements of the 7-ray yields from the T(D,y)"He and 3 He(D,-y) Li reactions at 16.7 MeV and 16.4 MeV, respectively. Since the y-ray branching ratios are known (though small; see Table 1),4' a measurement of the y-ray yield is also a measure of the total fusion-reaction yield. The Y emissions from the "mirror" D-T and D- 3 He reactions are nearly identical, because the nuclear force is charge symmetric, and the product "He and "Li nuclei have similar energy levels with very broad first excited states. 4 Section II below describes the concepts for the NOVA Upgrade and Alcator C-MOD 7-ray diagnostics. In order to successfully develop these diagnostics, we have established a comprehensive set of y-ray sources and detectors. The sources, both radioisotopes and nuclear reactions, provide y rays of known intensity over an energy range of 0.1 MeV to 17 MeV and are described in Section III. The detectors 2

4 include scintillation detectors and a high-resolution, High-Purity Germanium (HPGe) crystal, and they are described in Section IV. Together, these tools will be used to fully characterize and calibrate new -y-ray detection systems. II. -y-ray Diagnostics for NOVA Upgrade and Alcator C-MOD Measurement of the high-energy D-T y rays in the NOVA Upgrade experiment is the only practical way to determine the fusion burn history. The other fusion products cannot be accurately used for this purpose; the neutrons are significantly Doppler broadened, and the charged particles are stopped in the high-density target plasma. Since the burn width in the NOVA Upgrade experiment will be short (- 500 ps), the D-T -y rays can easily be separated from the neutrons because of their disparate arrival times at the detector. Table 2 illustrates this by listing several pertinent times for a detector at distance d from the target: the 14 MeV neutron arrival time at the detector; the spread (FWHM) in neutron arrival times due to energy Doppler broadening (AE ~ 177 T 1 (kev) calculated for a 10 kev ion temperature using the formula of Brysk 7 ); and the -- ray arrival time at the detector. The y rays have essentially zero Doppler broadening. For this measurement we have proposed either a -y-to-pair-to-cerenkov spectrometer or a -y-to-pair spectrometer with high time resolution (~ 20 ps). The former diagnostic is similar in spirit to that attempted by Lerche, et a0.. An unwanted "background" of -y rays may be generated by the 14 MeV neutrons undergoing inelastic (n,n'y) reactions in the hohlraum, target, 3

5 and stem. (In high Z materials, the (n,n'y) cross sections are typically about 2 orders of magnitude higher than (n,-y) cross sections.) It is essential to understand this contribution to the y-ray signal, which might amplify the burn signal but would make an absolute D-T yield calibration impossible. In the case of the Alcator C-MOD tokamak, a -y-ray diagnostic for measuring the high-energy -y rays from D-*He reactions does not require the fast time response needed for the NOVA Upgrade diagnostic, because the pulse is much longer (- 1 s). Neither does it require the superior energy resolution of germanium detectors, because the y-ray spectrum is naturally quite broad. (The 16.4-MeV -y is ~1.5 MeV wide.') Therefore we plan to deploy a "standard", large (6" x 6") NaI(Tl) scintillation spectrometer on Alcator C-MOD to measure the absolute D- 3 He reaction yield. This information will complement active (SBD 9 ) and passive (activationo and track detectors) charged-particle diagnostics. In the Alcator C-MOD experiment, capture of 2.5 MeV D-D neutrons in the structure will generate a considerable background of -y rays. Shielding against these lower-energy -y rays and against the neutrons will be crucial. We are designing a suitable shield, and once it is assembled we will test its performance using our Cockcroft-Walton fusion-product generator 1, which is a well-calibrated source of 2.5 MeV D-D neutrons. III. -y-ray Sources We use many different -f-emitting radioactive isotopes in our laboratory 4

6 as calibration standards. The most useful are 22 Na, 137CS, 5 4 Mn, and 6*Co, because they emit only single -y lines or well-separated lines that are easily resolved by standard Nal(Tl) scintillation detectors. Other sources (e.g. 1'3Ba and 22 0Ra) emit many 7-ray lines that can be resolved only with the HPGe detector. The sources listed above are limited to 7-rays below 3 MeV, far below the ~16.5 MeV lines from D-He and D-T reactions. Therefore we have also used an 24 Am/Be neutron source as an absolute source of 4.44-MeV y rays. These are generated by the reaction 9 Be(a, n) 1 2 0*, where the 12C* nucleus is in the first excited state, which decays by 4.44 MeV 7-ray emission. We calculated the y-ray intensity from this source using the 24 1 Am activity (10 mci), the number of a-"be reactions per 24 'Am a decay, and the fraction of reactions that produce the 120* first excited state. The second quantity is n/a c 7 x 10-5,12 and the third quantity (R,) was calculated from the yields of each 120 final state (0, 1, and 2). These yields were calculated using the cross sections of Ref. 12, and taking into account a-particle slowing down in beryllium using the stopping-power formulas of Ziegler 13. The final result was R,=0.57, in good agreement with the measured value of Using R,=0.57 gives an absolute intensity of 4 x 10" y rays per a decay. Because the 120* nucleus is extremely short lived (61 fs) 1, the 4.44-MeV 7 ray is quite broad. We therefore fit the measured spectrum to a function based on the physics of the 7-ray interaction in order to accurately determine 5

7 the total peak counts for an absolute efficiency measurement (Fig. 1). In order to test our -y-ray detectors at even higher energies, we use several nuclear reactions in our Cockcroft-Walton fusion-product generator 1 (see Table 1). Of particular interest to us is the 1 'B(p,-y) C reaction, which yields -y rays at 4.44, 11.5, and 16 MeV. The thick-target 7-ray branching ratios for "B(p,y) 12 C have been measured 15, so by measuring the corresponding a particles from the 'B(p,a)aBe reaction we establish the "B(p,y) 12 C reaction as a high-energy 7-ray source of well-known absolute intensity. Fig. 2 shows the p-"b 7-ray spectrum measured with a 3" x 3" NaI(Tl) detector while a pressed boron target was bombarded with a 170-kV, 25-IA proton beam (inset shows the a-particle spectrum). This beam energy is sufficient to access the p-l'b reaction resonance at 163 kev. Similar data were used to calculate the absolute peak efficiency for our HPGe detector at 4.44 and 11.5 MeV (Fig. 3). IV. 7-Ray Detector Characterizations We use four different 7-ray detectors in the laboratory: two "standard" 3" x 3" NaI(Tl) detectors, a 5" x 3" NaI(Tl) detector, and a 200-cms (50% relative efficiency) HPGe detector. We are using the sources described in Section III to fully characterize and calibrate these detectors. For example, Fig. 3 illustrates the absolute peak efficiency measured for the HPGe detector. 6

8 Acknowledgements Dr. Ramon Leeper and Dr. Carlos Ruiz of Sandia National Laboratory provided detectors used in this work. We acknowledge Dr. Ed Cecil of the Colorado School of Mines for helpful discussions, and we thank Dr. Fredrick Seguin and Cristina Borris for their careful reading of the manuscript. This work was supported by LLNL Subcontract B116798, U.S. DOE Grant No. DE-FG02-91ER54109, by U.S. DOE Fusion Energy Postdoctoral Fellowshipt, and by U. S. DOE Magnetic Fusion Energy Technology Fellowshipt. 7

9 References 1. S. S. Medley, F. E. Cecil, D. Cole, M. A. Conway, et al., Rev. Sci. Instrum (1985). 2. G. Sadler, 0. N. Jarvis, P. v. Belle, et al., in Controlled Fusion and Plasma Heating (Proc. 14th Eur. Conf., Madrid, 1987), 11D, Part III, p (1987). 3. R. D. Petrasso, C. L. Fiore, and C. K. Li, Rev. Sci. Instrum (1988). 4. J. E. Kameraad, J. Hall, K. Sale, et al., submitted to Phys. Rev. C (1991). 5. F. E. Cecil, D. M. Cole, R. Philbin, Phys. Rev. C (1985). 6. F. E. Cecil and F. J. Wilkinson III, Phys. Rev. Lett (1984). 7. H. Brysk, Plasma Phys (1973). 8. R. A. Lerche, M. D. Cable, D. W. Phillon, Rev. Sci. Instrum (1990). 9. D. H. Lo, R. D. Petrasso, K. W. Wenzel, et al., submitted to Rev. Sci. Instrum. (1992). 10. J. R. Lierzer, R. D. Petrasso, K. W. Wenzel, et al., submitted to Rev. Sci. Instrum, (1992). 8

10 11. K. W. Wenzel, D. H. Lo, R. D. Petrasso, et al. submitted to Rev. Sci. Instrum., (1992). 12. K. W. Geiger and L. van der Zwan, Nucl. Instrum. and Meth (1975). 13. J. F. Ziegler, Helium: Stopping Power and Ranges in all Elemental Matter, Pergamon Press, (1977) 14. S. Croft, Nucl. Instrum. and Meth. A (1989). 15. F. E. Cecil, D. Ferg, H. Liu, et al., to be published in Nucl. Phys. A (1992). 16. R. Gunnink, Nucl. Instrum. and Meth. A (1990). 17. F. E. Cecil and S. S. Medley, DOE-APP alpha particle workshop, Germantown MD, Dec. (1989), unpublished. 9

11 Table 1: Ground-state fusion reactions that generate high-energy -y rays. Reactants Particle Q -t-ray E., -y-ray Refs. I Branch (MeV) Products (MeV)j B.R. p+d oo 1 p+t - - f+a 19.8 o 1 p+5li YHe+a 4.02 Y+"Be x p+7li a + a 'Be x p+ 9 Be a+ali 2.12 y+ 10 B x p+"b c+sbe 8.59 ' C x 10-r 15 3a 8.68 D+D p+t 4.03 Y + a x n+he 3.27 D+T n+a He x 10-6,1 D+ 3 He p+a y+ 5 Li x 10" 5 D+ 6 Li a + a Y+'Be < 10-g 17 D+ 7 Li a+ 5 He Be < 7 x 10-B 17 10

12 Table 2: For a 10-keV ICF plasma, this table compares the arrival time (t") of the 14-MeV neutrons, the spread (FWHM) in their arrival times (At), and the arrival time (t.) of -y rays at a detector located a distance d from the target. A burn width -500 ps is expected in the NOVA Upgrade. d t" At t (cm) I(ps) I(ps) (ps) 10 1, , , , ,

13 'U U TI 24 1 Amn/Be 100 -: Energy (kev) - - o -o 6 0 *. Wool' *-...*..'..*-* ' '..' I'' Energy (kev) Fig. 1. The y-ray spectrum from a 10 mci 24 1 Am/Be source was obtained with our 200-cm 3 HPGe detector. The portion of the spectrum around the 4.44 MeV peak (inset shows the entire spectrum) was fit to the function a, e-(z-2/a+ a4e-(b-aj+ a 7 E +as. In this fit, the low-energy gaussian represents the Compton edge, and the high-energy gaussian represents the full-energy peak. Total counts derived from this fit were used to determine the absolute peak efficiency of the HPGe detector at 4.44 MeV (Fig. 3). 12

14 1 5 "3 ' B(p,a)2a 104:_ 200 C -01U. ISO JC oso50 I B(p,a)880' , Energy (MOV) L M o Energy (MeV) Fig. 2. A p-l'b 7-ray spectrum was taken with a 3" x 3" NaI(TI) detector over two hours while we bombarded a boron target with a 170-kV, 25-pA proton beam (inset shows the corresponding a-particle spectrum). Similar data were used to measure the absolute peak efficiency of the HPGe detector at 4.44 and 11.5 MeV (Fig. 3). 13

15 10-1 d=10 cm a1 = : a 2 = C a 3 = a 4 = a5= a -~ a 10-3 CL eeti y:.sna :: Energy (MeV) Fig. 3. Absolute peak efficiency (peak counts per source photon) was measured for our HP Ge detector at a source-to-detector distance of 10 cm for y energies from 0.19 to 11.5 MeV (data points; the 4.44 MeV point from p-" 1 B reactions is offset to 5.44 MeV for clarity of display). We fit these data to a function, ln(r1) = E?.a[ln(E)](" 1 ), a common form for solid state detectors for energy >200 kev. 1 The uncertainties in these data were derived by adding in quadrature the uncertainties in absolute source intensity, geometry, and counting statistics. 14

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