The Pentagon has found that because of technical reasons it cannot, as once planned, build neutron shells for the most widely deployed NATO artillery piece, the 155mm howitzer, according to government sources.

Thus, if President Carter should eventually order a go-ahead for production of controversial neutron weapons, the United States would build fewer than half the number originally programmed, sources estimate yesterday.

Last year the Pentagon included the 155mm shell in the new neutron family of tactical nuclear weapons, which also encompassed the 8-inch artillery shell and the Lance missile warhead.

Neutron versions of the 8-inch shell and the Lance warhead were designed, tested and ready for production by last summer, when the controversy over the weapons first became public.

Research on the 155 mm version was about a year behind that for the other two.

In numbers, however, the 155s were to be the major part of the neutron arsenal.There are 1,290 155 mm howitzers among the NATO forces in Europe. There are 450 8-inch guns and 92 Lance launchers programmed for the theater, according to published congressional testimony.

On April 7 when President Carter held up immediate production of the neutron 8-inch and Lance shells, the engineering research on the 155 shell was just being approved.

By that date, however, Department of Energy scientists, who develop and build U.S. nuclear weapons, had found that they could not miniaturize the nuetron components to fit into the small 155mm shell.

Therefore, the Pentagon this spring quietly approved building a new 155mm nuclear shell, but of the traditional fission variety.

The new 155, and the 8-inch are to replace almost 20-year-old nuclear shells deployed in Europe.

Even within the neutron feature, the new shells would provide twice the 10-mile ranges of the older ones and would include new safety features that allow them to be rendered harmless if stolen. They also would include a security device that would require a special numerical code to arm them.

Nuclear weapons now deployed depend primarily on blast and heat effects, while neutron versions produce most of their killing yield in radiation.

The move to neutron weapons has been justified in part beacuse they would sharply limit the amount of colateral damage to areas outside the immediate target area.

When it was found the 155s could be made neutron, sources said, DOE scientists moved ahead with a version that would almost double the blast and heat yields of the currently deployed 155mm shells.

"They forget the collateral damage idea," one source noted, "and justified building it to its powerful new blast effect."

Meanwhile, the Pentagon is still wrestling with what to do with the 8-inch and Lance missile shells in complying with Carter's April 7 directive to move ahead with their "modernization" but not make them neutron.

Some officials want to build the neutron versions but keep the neutron cores separate - a step that would provide a low-yield nuclear weapon but keep the opportunity to quickly convert them.

Others say that would violate the president's decision to defer production. They want to build the low-yield versions and delay production of the cores for a year to see if the Soviets respond in an acceptable fashion.

A Pentagon spokesman yesterday said that a recommendation to the president, once promised for April 21, is hopefully going to the White House next week.