tokens: of the ion spectra, here taken to be 30 MeV nucleon −1 . The figure suggests that neutron production is more effective for steep ion spectra, and this is true but only if the number of ions above this normalizing energy is kept constant as the index is changed. Figure 6. Total angle-integrated neutron yield, , as a function of the accelerated-ion spectral index for accelerated α /proton = 0.1 and 3 He/ 4 He = 0.0, 0.5, 1.0, 5.0, and 10.0. Even for accelerated 3 He/ 4 He ratios as large as 1, neutron production is seen to be increased only by about a factor of 2. Only for much larger 3 He enhancement factors would the neutron yield be significantly affected. Most of the impact is for ion spectral indexes near the middle of the index range (3–6). We can understand this by considering Figure 7 , showing the fraction of the total neutron yield contributed by each of the 11 reaction types (p+ 4 He, 4 He+H, p+H, 4 He+ 4 He, p+[CNO], [CNO]+H, 4 He+[CNO], [CNO]+ 4 He, 3 He+H, 3 He+ 4 He, and 3 He+[CNO], where “[CNO]” refers to all nuclear species heavier than 4 He). For spectral indexes of 3–5, the 3 He + H reaction contributes more than 10% of the total (and the new 3 He + [CNO] reactions, shown in red, are not significant). For flatter spectra, neutron production is dominated by proton reactions so the impact of enhanced 3 He is less. For steeper ion spectra, the contribution of the 3 He + H reaction falls due to its relatively high threshold energy and the 3 He + [CNO] reactions increase to contribute up to about 10%. We note that for steep spectra, the contribution of the 4 He + [CNO] reactions and their inverse reactions [CNO] + 4 He are both strong. As noted above, the corresponding 3 He inverse reactions ([CNO] + 3 He) are not included because of the very low abundance of 3 He in the ambient solar atmosphere ( 3 He/H ∼ 10 −5 ). Figure 7. Fraction of the total neutron yield contributed by the 11 reaction types for accelerated α /proton = 0.1 and 3 He/ 4 He = 0.5. The red curve shows the fraction from 3 He + [CNO] reactions. Figure 8 shows the total angle-integrated neutron yield as a function of accelerated-ion spectral index for two values of accelerated α /proton: 0.1 and 0.5. For each value, total yields are shown for (1) coronal accelerated-ion composition and 3 He/ 4 He = 0 (solid), (2) coronal accelerated-ion composition and 3 He/ 4 He = 0.5 (dotted), and (3) impulsive-flare accelerated-ion composition and 3 He/ 4 He = 0.5 (dashed). For the α /proton = 0.1 case, the impact of the impulsive-flare accelerated-ion composition via the inverse reactions can be seen for the steeper spectra because the cross sections for neutron production by α -particle (and 3 He) reactions with heavy elements tend to have lower threshold energies than those of proton reactions; these reactions are emphasized by steep accelerated-ion spectra. The impact of the impulsive-flare accelerated-ion composition is not as large for the α /proton = 0.5 case because
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