I'm guessing he was working on an experiment where the proton beam exits the vacuum chamber and interacts with a sample and some detectors outside of a vacuum.

I'm guessing he was working on an experiment where the proton beam exits the vacuum chamber and interacts with a sample and some detectors outside of a vacuum. https://www.isis.stfc.ac.uk/Pages/Chipir-technical-information.aspx https://www.isis.stfc.ac.uk/Pages/Chipir-technical-informati... shows an example where a neutron beam exits into a shielded block house where experiments can take place. In this modern particle accelerator access to the block house is controlled by digital and mechanical interlocks to prevent such an accident occurring.