Attorneys for Samuel L. Popkin, assistant professor of Government, will argue their appeal of his March 29 contempt finding mainly on grounds of protection under the First Amendment.

William P. Homans Jr. '41 and Daniel Klubock, Popkin's attorneys, filed their 56-page brief yesterday for an April 19 hearing before the First U.S. Circuit Court of Appeals.

Popkin was found in civil contempt for refusing to answer nine questions before the Boston grand jury investigating the Pentagon Papers case. That finding, by Federal District Court Judge W. Arthur Garrity Jr., was made without regard to the First Amendment question.

Popkin has argued since his first grand jury subpoena last August that he should not be required to answer questions relating to sources of information used in his scholarly research on the grounds that this would violate his First Amendment rights.

The major arguments put forth in the brief by Popkin's attorneys are:

* that a "scholar, author and teacher has a privilege not to divulge the confidential sources of his information":

* that Popkin's claim to privilege closely parallels the Supreme Court's findings in Caldwell vs. United States, which held that a journalist's sources are protected:

* that affidavits from other scholars--including James Q. Wilson, chairman of the Government Department: John D. Montgomery, professor of Public Administration: Edwin O. Reischauer, University Professor: John Kenneth Galbraith, Warburg Professor of Economics: and John K. Fairbank, Higginson Professor of History--support the contention that protection of scholarly sources is necessary:

* that the questions in dispute were not relevant to the stated goal of the grand jury investigation; and.

*that Popkin was entitled to be advised of the relevance of questions at his request.

The brief also claims that Garrity should not have denied a motion on Popkin's behalf that the government be required to file an affidavit specifying whether any electronic surveillance had been used to intercept telephone or oral conversations.

Popkin, at the beginning of his grand jury appearance, stated that he had no knowledge of plans to publish or distribute the Pentagon Papers. Despite this, the brief--citing another court decision--says. "The Assistant United States Attorney embarked on a fishing expedition with questions, the answers to which might reveal further sources of information which, if pursued, might eventually lead to a pertinent inquiry."

"It is difficult to think where pertinency would not exist under such a definition," the case cited said.