LASL INTENSE 14-MeV NEUTRON SOURCE.

At least two independent suggestions have been made to build a 14-MeV neutron source at 10* to 10 neutrons/cm-sec for materials damage studies related to fusion power reactors. Greater detail of the LASL suggestion is presented here than in our earlier report. A feasibility study is proposed in which such a 14 MeV source would be thoroughly investigated. :. INTRODUCTION The need for neutron sources suitable to study Lamage phenomena anticipated in fusion reactor itructural materials has been, by now, well estabished. The latest estimate from the Scyllac group, or instance, is that the wall of a prototype oulsed i-T reactjr would sustain a flux of 14.06 MeV neuxons equal to 2.7 x 10* neut/cm -sec (at one pulse ier second). * Other estimates span the range from 0 to 10 l s neut/cm -sec at 14 MeV. There are sevra. anticipated and perhaps also some unexpected lamage mechanisms which will be important at such luxes and energies. The microscopic damage effects transmutations to other metals, transmutation to ielium and hydrogen, production of lattice vacancies nd interstitials, etc.) will interact with the host iaterial, with each other, and with effects due to he material temperature and stress fields in a very omplicated way. The resulting effects of engineerng interest swelling, reduced ductility, enhanced xeep rates, modified strength, etc. are expected to be very important, but are impossible to predict from knowledge of the microscopic effects. It is for this r«ason that a neutron source is needed which can provide the correct flux and energy spectrum to samples which may be maintained at the appropriate temperature under the appropriate stress loading. Such a source is quite obviously a variant of the Cockcroft-Walton D-T source, in continuous operation at a thousand times the present day maximum intensity, with enough room available close-in to locate the experimental samples. Such a source was proposed for fusion reactor application by Colombant and Lidsky and then again in the present context at LASL by Dreicer and Henderson. The present report summarizes the ideas which underlay our earlier report with some improvements in the calculations. It also locates certain LASL expertise and facilities which could be brought to bear on a program to actually build such a source. These include extensive special experience with neutrons, hot high pressure hydrogen, fluid mechanics computation, accelerators, tritium, radiochemistry, and metallurgy. Finally, it suggests a short program which would establish the feasibility of the project, establish a reasonably firm design from among the possible variations, establish a good cost estimate including the location of suitable equipmei t available as government surplus, and work out a . subsequent program which could be followed to the completion of the then proposed facility. II. CONCEPT The generation of 14 MeV neutrons from the DT reactions using a Cockcroft-Walton machine is a standard technique. In general a beam of deuterons is stopped in tritium-in-metal target such as developed at Los Alamos. * The specific yield from such targets is about 3 x 10 neutrons/joule and their intensity is limited by target heating. If one assumes that he might cool the target sufficiently well to retain the tritium loading at 3 kW/cm (1.1 kW/cm has been achieved), the maximum source intensity is (3 x 10) x (3 x 10 )«10 1 2 neut/cm-sec. The present idea is to use a windowless gas target in order to gain a factor of 10 in specific yield (neutrons/joule) and a factor of 100 in allowable beam power density. This is an old idea, mentioned as hiving been attempted without success in reference 5. Recently, however, a windowless gas target has been made to work at Los Alamos, for a different application. When a current T_ of ions of one of the heavy hydrogen isotopes traverse • gas at density n of the other, most are simply slowed down, losing energy E as dC = -n e(E) dX, where e is the stopping power of the gas for the ion in units of (say) keVbarn;;. A small fraction will, however, produce neutrons N as dN = In n a(E) dX = -I B [a(E)/e(E)] dE, jjhere a is the cross section in barns. Standard data' for e and a are displayed for both the D + beam case: T(d,n)a and the T beam case: D(t,n)a in Figs. 1 and 2. The differential yield, a/e in neutrons/joule, displayed in Fig. 3, is integrated to find the yield out J dE, E. in The range or required target thickness is -out R = J [l/e(E)] dE,atoms in The yield Y and range R for the case of a thick target (E = 0) are displayed in Figs. 4 and 5. [Note that we may equate the ion range and the target thickness or projected range since the difference is only (m /3.5 m.) = 0.005%. ] As has already been alluded to, the experimental difficulties are most stro igly dependent upon the beam and target power. Therefore the specific yield, Y /E. , which is plo*ted in Fig. 6 is of special interest. Notice that there is no efficiency advantage to accelerating either species ovar the other: a maximum of 3 x 10 neutrons/joule is available either way. One chooses therefore to accelerate the tritium in order to minimize the tritium inventory and the associated costs and hazards. One further notes that the maximum is very broad; the range E i n = 250 keV to 370 keV has a specific yield within 100 200 300 ION ENERGY. E, k«V Fig. 1, Stopping power for D in T, and for T in D,. 400 100 200 300 ION ENERGY, E, k«V Fig. 2. Neutron production cross section. 400 100 200 300 BEAM ENERGY, E^,, k«V 400 Fig. 4. Thick target yield.