[HN Gopher] Physicists make most precise measurement ever of neu...
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       Physicists make most precise measurement ever of neutron's lifetime
        
       Author : mkoc
       Score  : 24 points
       Date   : 2021-10-18 21:17 UTC (1 hours ago)
        
 (HTM) web link (www.nature.com)
 (TXT) w3m dump (www.nature.com)
        
       | thriftwy wrote:
       | Stupid question, do they account for speed dilation on the beam
       | experiment? The difference in lifetime will translate to 50k km/s
       | speed of neutrons or roughly 1/6c
        
         | perihelions wrote:
         | From skimming the review paper from OP (the "source:" in the
         | caption on that error-bar chart), the neutrons in the beam
         | experiments are thermalized, to a mean velocity of ~2,200 m/s.
         | So, slower than 1e-5 c.
         | 
         | https://doi.org/10.3390/atoms6040070
         | 
         | (Thermal meaning the neutrons scatter lots of times against
         | atoms in a solid material, until they reach thermal
         | equilibrium. ~km/s is a typical Boltzmann velocity for atom-
         | size things at room temperature).
         | 
         | (Not a domain expert).
        
       | davidhyde wrote:
       | > Neutrons in beams seem to live longer on average
       | 
       | Isn't this due to relativistic effects? What percentage of the
       | speed of light are these beams?
        
         | fsh wrote:
         | Neutron lifetime experiments are done with cold beams (tens of
         | K). Time dilation is completely negligible.
        
           | [deleted]
        
           | [deleted]
        
       | liquidise wrote:
       | > [isolated neutrons] decay into protons. During the process,
       | each decaying neutron emits an electron and an antineutrino.
       | 
       | > ...detected sparks of light each time a neutron decayed.
       | 
       | Detecting a spark of light would also require photon(s) to be
       | emitted, right? Is this not called out because it is a byproduct
       | of the decay and not part of the decay reaction itself?
        
         | sharikous wrote:
         | No, they don't measure decays.
         | 
         | They let the neutrons decay for a while and then measure all
         | the remaining neutrons in the trap by lowering there a
         | detector.
         | 
         | This detector has a scintillator so when a neutron is captured
         | it emits some photons and those are converted to an electric
         | signal.
         | 
         | So no. There is no requirement for the neutron to emit an
         | additional photon during the decay that is measured
        
         | LatteLazy wrote:
         | Yes, the decay will also release some energy in the form of
         | photons but they didn't mention it.
        
           | sharikous wrote:
           | That's inaccurate, please see my reply to the same comment
        
           | a1369209993 wrote:
           | Neutron decay does not produce photons. Rather, the energy
           | difference is carried as kinetic energy by the resulting
           | proton, electron and (anti)neutrino. This _can_ cause light
           | to be emitted (very shortly) _later_ when those particles
           | slam into things, but as sharikous notes that doesn 't seem
           | to be they're talking about here.
        
         | [deleted]
        
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