Mainly a high Z, because heavy nuclei have a neutron excess. So destroying the nuclear structure, the yield of free neutrons (per mass or per process) is higher if Z is higher. 

The number of spallation neutrons per incident proton depends on the beam energy and on the mass of the target nuclei. Due to their high atomic number, heavy metals such as lead (Pb), mercury (Hg), uranium (U), tungsten (W), or eutectics such as lead-bismuth (Pb-Bi) are the most appropriate choices for the target material. Moreover, because of the requirement to minimize the size of the target in relation to the core dimensions (to provide good neutron economy); the target design is driven to maximum power density. For the very high power densities reached in a spallation target (several hundred kW per liter for proton beam powers of several MW), a flowing liquid metal target material provides the best option for practical removal of the heat by convection. The use of a heavy liquid metal (HLM) also significantly reduces the damage caused by intense radiation to the target itself and the structural materials.

Lead-bismuth eutectic (LBE) is today the reference target material for accelerator demonstration systems (ADS) applications. Both lead and lead-bismuth exhibit very low neutron capture making them good candidates from a neutronic standpoint. Lead might at first appear a better target choice than LBE, because of its considerably lower production of (polonium-210 (210Po) a migratory alfa-emitter under neutron bombardment. However, lead has the critical disadvantage of a higher melting point (327°C compared with 123.5°C), which presents severe thermo-mechanical design challenges for the target structural materials. The 210Po release problem in LBE is mitigated by the fact that Po forms a chemical bond with Pb to form metastable Pb-Po, which has an emanation rate (vapour pressure) 1 000 times lower than that of Po by itself.

The main purpose of the high power spallation target in ADS is to provide the primary neutron flux for driving the fission process in the surrounding subcritical core. For this reason the neutron yield is an important consideration in selecting the energy and current parameters of the proton beam. Experimental data show that in the energy range relevant for ADS (300-1 000 MeV) the neutron yield per incident proton (n/p) scales approximately as Eproton 3/2 (with Eproton in MeV). Thus, for a given neutron yield, lower beam current is required at higher beam energy. An advantage of higher beam energy is greater penetration depth in the target, leading to lower axial power densities. The reduced relative importance of the Bragg peak (compared with meson production) shifts the beam heating profile towards the target surface, but does not cause a problem except at lower energies.

Although the energy deposition at the surface per proton increases at higher energies, this effect is compensated by the reduced current requirement. In addition, a lower fraction of the total beam power is deposited as heat in the target at higher proton energies. However, a penalty for higher beam energy and lower current may be increased accelerator cost.