New atmosphere for beamline target chase

The LBNF beamline design originally called for an air-filled/air-cooled target chase, the enclosure in which the target and horns are installed, and for air-cooling of the decay pipe. The older NuMI beamline, however, which also used air, brought to light some issues that the LBNF team needed to consider: the production of corrosive chemicals and air-borne radioisotopes.

The LBNF beamline project has selected nitrogen as the atmosphere for the target chase. This is a big – and well motivated — change for LBNF.

A model of the LBNF beamline. The target chase is shown in cyan. The beam enters from the left, hits the target, and traverses the horns (silver) before entering the cylindrical decay pipe. The target chase hatch covers are shown in white, above the shielding. Credit: LBNF

“Given LBNF’s higher projected beam intensity, and that the production of radioisotopes is proportional to the beam power, use of a design similar to NuMI’s might not remain within safety limits,” said Fermilab scientist Alberto Marchionni. “And corrosion of elements inside the chase would likely be a larger concern.”

The NuMI beam creates radioisotopes in the air, 11C, 13N and 41Ar. The first two have half-lives under a half hour, and mostly decay before release.  But 41Ar has a longer one, nearly two hours. The beam also produces radioactive tritium (T) in the target chase, which quickly combines with other elements and is released as hydrogen gas (HT, of low biological hazard) and as tritiated water vapor (HTO).  In the case of NuMI, the cumulative radioisotope release remains within safety limits.

Fermilab physicists Kamran Vaziri, Jim Hylen and Peter Kasper ran radiological calculations that pointed toward replacing the air in the LBNF target chase and the decay pipe cooling system with a different gas. The project has just now made the decision to use nitrogen.

“This took a lot of thinking,” said Marchionni, who originally forwarded the idea of using nitrogen. “We’ve been working on this for about three years.”

Lacking argon, a nitrogen atmosphere prevents production, and thus release, of 41Ar, the longer-lived radioisotope. Lacking oxygen and natural humidity, it also prevents formation of HTO.  The lack of oxygen and humidity also mitigates the risk of corrosion.

To prevent buildup of 3H in the chase, the design calls for a slow release and replacement of the nitrogen.

“Given the volume turnover rate of about a week, the short-lived radioisotopes 11C and 13N in the released gas should decay to safe levels by the time the gas transits along its path to its release point near the upstream end of the primary beamline,” said Hylen.

Among several design changes required to contain the nitrogen, a series of hatch covers on the target chase underwent a complete – and challenging — redesign to provide nearly hermetic isolation of the chase. The seal has to be just right to keep the nitrogen turnover rate slow and steady – and to still allow the hatches to be opened and closed.

Engineers Salman Tariq and Andy Stefanik led the technical design team, and major contributors included Jim Hylen as well as Joe Angelo, David Pushka and Matthew Sawtell, all of Fermilab.

“We have completed the conceptual design for the hatch cover sealing system together with the hermetic feedthroughs and a nitrogen fill/purge system, but we still have to validate the technical designs” said Tariq. “During the preliminary design phase we plan to conduct prototype tests for the different sealing systems.”