TS2 on Target – view from the LOQ cabin

A brief overview of some neutron history

Since it carries no charge, one could not “weigh” a neutron directly in a mass spectrometer, but had to estimate its mass from the difference between deuterium and hydrogen. However in 1935, more accurate measurements allowed Chadwick to derive a neutron mass of between 1.0084 and 1.0090 units; the modern estimate lies almost exactly in the middle of this range, and so the neutron is appreciably heavier than the proton. Was Uncle Albert wrong? Chadwick immediately suggested (following Einstein again) that neutrons should have an excess of energy and be beta radioactive in common with other nuclei under like circumstances. It was not so easy, however, to verify this experimentally. But, partly owing to World War II, it was not until 1948 that neutron decay was verified – indeed, 12 minutes after they’ve been kicked out of the nucleus half of them will have split apart as Chadwick had suggested. So if you want to use neutrons, you can’t keep ’em in a bucket.

That last bit is a tad misleading — it’s no doubt referring to the decay, not the ability to confine. Since neutrons have a magnetic moment, they can be trapped. It’s a weird, shallow bucket, and it leaks, but you can keep ’em there until they decay.

From our “Plan of the Week”
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August 12 & 17 1877 – Asaph Hall discovers the moons of Mars. From Halls’ notebooks: I repeated the examination in the early part of the night of [August] 11th, and again found nothing, but trying again some hours later I found a faint object on the following side and a little north of the planet. I had barely time to secure an observation of its position when fog from the River stopped the work. This was at half past two o’clock on the night of the 11th. Cloudy weather intervened for several days.

On 15 August the weather looking more promising, I slept at the Observatory. The sky cleared off with a thunderstorm at 11 o’clock and the search was resumed. The atmosphere however was in a very bad condition and Mars was so blazing and unsteady that nothing could be seen of the object, which we now know was at that time so near the planet as to be invisible.

On August 16 the object was found again on the following side of the planet, and the observations of that night showed that it was moving with the planet, and if a satellite, was near one of its elongations.

Until this time I had said nothing to anyone at the Observatory of my search for a satellite of Mars, but on leaving the observatory after these observations of the 16th, at about three o’clock in the morning, I told my assistant, George Anderson, to whom I had shown the object, that I thought I had discovered a satellite of Mars. I told him also to keep quiet as I did not wish anything said until the matter was beyond doubt.

He said nothing, but the thing was too good to keep and I let it out myself. On 17 August between one and two o’clock, while I was reducing my observations, Professor Newcomb came into my room to eat his lunch and I showed him my measures of the faint object near Mars which proved that it was moving with the planet. On August 17 while waiting and watching for the outer moon, the inner one was discovered. The observations of the 17th and 18th put beyond doubt the character of these objects and the discovery was publicly announced by Admiral Rodgers.
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This was from a time when the Observatory was located at Foggy Bottom — it was about 16 years later that it was moved to its current location. But the same telescope is still in use.

(And it’s probably a good thing Hall didn’t discover the moons as a university professor, because then the Astronomy building named after him would be Asaph Hall Hall)