"clean" tests used a lead pusher, while in 1958 a tungsten carbide pusher was employed. Hans A. Bethe supported clean nuclear weapons in 1958 as Chairman of a Presidential science advisory group on nuclear testing: ... certain hard targets require ground bursts, such as airfield runways if it is desired to make a crater, railroad yards if severe destruction of tracks is to be accomplished ... The use of clean weapons in strategic situations may be indicated in order to protect the local population.– Dr. Hans Bethe, Working Group Chairman, 27 March 1958 "Top Secret – Restricted Data" Report to the NSC Ad Hoc Working Group on the Technical Feasibility of a Cessation of Nuclear Testing, p 9. In consequence of Bethe's recommendations, on July 12, 1958, the Hardtack-Poplar shot of the Mk-41C warhead was carried out on a barge in the lagoon yielded 9.3 megatons, of which only 4.8% was fission, and thus 95.2% "clean". In 1958, Cohen investigated a low-yield "clean" nuclear weapon and discovered that the "clean" bomb case thickness scales as the cube root of yield. So a larger percentage of neutrons escapes from a small detonation, due to the thinner case required to reflect back X-rays during the secondary stage (fusion) ignition. For example, a 1-kiloton bomb only needs a case one-tenth the thickness of that required for 1-megaton. So, although most neutrons are absorbed by the casing in a 1-megaton bomb, in a 1-kiloton bomb they would mostly escape. A neutron bomb is only feasible if the yield is sufficiently high that efficient fusion stage ignition is possible, and if the yield is low enough that the case thickness will not absorb too many neutrons. This means that neutron bombs have a yield range of 1–10 kilotons, with fission proportion varying from 50% at 1-kiloton to 25% at 10-kilotons (all of which comes from the primary stage). The neutron output per kiloton is then 10–15 times greater than for a pure fission implosion weapon or for a strategic warhead like a W87 or W88. U.S. Department of Defense manual on the neutron bomb Cohen's neutron bomb is not mentioned in the unclassified manual by Glasstone and Dolan, The Effects of Nuclear Weapons 1957–1977, but is included as an "enhanced neutron weapon" in chapter 5 of the declassified (formerly secret) manual edited by Philip J. Dolan, Capabilities of Nuclear Weapons, U.S. Department of Defense, effects