What are the "beyond expectation" events at J-PARC in which radioactive materials leaked after they managed to vaporize gold by hitting it with proton beam that was a bit too strong?
They say (according to Jiji Tsushin, 5/25/2013):
They didn't even imagine that the equipment could malfunction; and
They didn't even imagine that proton beam could be so strong that it vaporized the metal; and therefore,
They didn't even imagine that radioactive materials, which in normal situation remain inside the metal, would leak outside the equipment; therefore,
The ventilating fan did not have a filter to block the leak of radioactive materials.
NHK's latest news adds more "beyond expectation" events:
They did check the surface contamination on May 23, the day of the accident, and it showed the contamination 10 times the normal level. But they didn't even suspect internal radiation exposure, and let the researchers go home;
They didn't check the researchers for internal radiation exposure until 4 researchers asked to be tested;
They are yet to test 55 others who at the facility.
The highest radiation exposure, of both internal and external radiation, was 2 millisieverts, according to NHK.
NHK also says they noticed there was a leak of radioactive materials when the radiation level in the facility dropped after using the ventilation fan.
From the tweet of one of the nuclear researchers that I follow (Tomohiro Endo, of Nagoya University), "Calculation of distribution of all nuclides (including stable isotopes and radionuclides) created when 30GeV proton beams hit 1mm-thick gold (30GeVの陽子を1mm厚さの金に当てたときに生成される全核種の分布を計算)":
(Click to enlarge)
He says in another tweet that they are the sum of (1) nuclides created when protons hit gold, and (2) nuclides created when neutrons hit gold.
Most likely radionuclides from the accident are in red and yellow, which he thinks include tritium (proton number 1, neutron number 2) - the orange dot in the lower left corner, and radioactive gold (Au-196), on the left of the orange dot which is stable AU-187 in the upper right corner.
Au-196 has half life of about 6 days. Half life of tritium is about 12 years.