I have been curious about that as well, as there does not appear to be anything precise about what Slotin was doing. The impression I have from what I have read so far was that he was still in the set-up stage of the experiment, which required an assembly that was close to criticality, and I'm guessing that, once that had been achieved, that actual experiment would be conducted with much greater precision. In this case, he was merely demonstrating the procedure to Alvin Graves, who was to take over Slotin's work, so there may have been no intention of making precise measurements.
Raemer Schreiber, who was present in the lab working on something else, wrote "At first Slotin said that he didn't have the proper materials for one. Then he remembered that we had the 49 cores there so he said he would do one 'in about two minutes' in a beryllium tamper after we (Schreiber and Perlman) had finished our counts. I remarked that if he were going to do it in two minutes I was going to leave but would stick around if he took a half hour for it. This was not intended seriously since we all had confidence in Slotin's ability and judgment." In the context of what else I have read about the experiment, I think the "about two minutes" comment referred to how long it would take for Slotin to set up for the experiment.
He also wrote "I had assumed that the approach to critical would be rather slow so continued to work on the initiator, thinking that when the multiplication got to an interesting point I would turn and watch." A near- but sub-critical assembly serves as a multiplier (amplifier) of any neutron flux it is exposed to, with the gain asymptotically approaching infinity as the assembly approaches criticality. Again according to Schreiber, Slotin was using a neutron-producing "driving source", and my guess is that setting up for the experiment involved adjusting the assembly until it was producing a sufficiently large multiplication of the driver's neutron flux.
As to why this experiment had to be repeated, I get the impression that, at this stage, each bomb was custom-built (or at least the ones for the Bikini tests were), and its fissile core and the surrounding tamper had to be individually calibrated and adjusted.
Beyond the description of what Slotin was doing at the time of the accident, I have not seen any description of how the experiment was performed in terms of what measurements were taken, or of its precise purpose.
Raemer Schreiber, who was present in the lab working on something else, wrote "At first Slotin said that he didn't have the proper materials for one. Then he remembered that we had the 49 cores there so he said he would do one 'in about two minutes' in a beryllium tamper after we (Schreiber and Perlman) had finished our counts. I remarked that if he were going to do it in two minutes I was going to leave but would stick around if he took a half hour for it. This was not intended seriously since we all had confidence in Slotin's ability and judgment." In the context of what else I have read about the experiment, I think the "about two minutes" comment referred to how long it would take for Slotin to set up for the experiment.
He also wrote "I had assumed that the approach to critical would be rather slow so continued to work on the initiator, thinking that when the multiplication got to an interesting point I would turn and watch." A near- but sub-critical assembly serves as a multiplier (amplifier) of any neutron flux it is exposed to, with the gain asymptotically approaching infinity as the assembly approaches criticality. Again according to Schreiber, Slotin was using a neutron-producing "driving source", and my guess is that setting up for the experiment involved adjusting the assembly until it was producing a sufficiently large multiplication of the driver's neutron flux.
As to why this experiment had to be repeated, I get the impression that, at this stage, each bomb was custom-built (or at least the ones for the Bikini tests were), and its fissile core and the surrounding tamper had to be individually calibrated and adjusted.
Beyond the description of what Slotin was doing at the time of the accident, I have not seen any description of how the experiment was performed in terms of what measurements were taken, or of its precise purpose.
https://www.halflifeofgenius.com/slotin - thanks to toomuchtodo for the reference.