Perkeo III is an instrument to perform precision experiments on the weak interaction. As a major improvement compared to its predecessor Perkeo II, the decay volume is considerably larger and accepts a large fraction of the natural beam divergence. This makes measurements with a pulsed neutron beam possible without losing too much statistics. This effecively eliminates leading sources of systematic error: background and solid angle effects. Neutron beam dependent background is measured under the same conditions as the neutron decay signal itself and thus fully subtracted.

Like all PERKEO spectrometers, PERKEO III uses a magnetic field to transport electrons and protons from the decay region towards the detectors in a symmetric set-up, ensuring a solid angle acceptance of (almost) 4π. The magnetic field also serves as quantisation axis for the neutron spin. Both detectors are connected by the magnetic field so that even in the case of electron backscattering their full energy will be detected.


The Perkeo III instrument was built by the Physikalisches Institut of the Universität Heidelberg. It was commissioned successfully in a first run at the Institute Laue-Langevin (ILL) in 2007. This experiment used a continous neutron beam and obtained an unprecented detected neutron decay rate of 50.000 s-1 within the spectrometer, two orders of magnitude more than in previous experiments! The results of this measurement have been published in (Märkisch et al., 2009) and some impressions from this first beam time and the installation at the cold neutron beam line PF1b at the ILL are online.

In 2008/2009 we performed a measurement of the beta asymmetry parameter A at the Institut Laue-Langevin. In contrast to the first run, a pulsed neutron beam is used. Data is only taken while the neutron pulse is fully contained within the central volume of the spectrometer and thus leading sources of systematic uncertainty are eliminated:

  • The projection of the decay products ion the detectors is now edge-free and no solid angle corrections are required.
  • The background from the collimation system etc. can be fully measured and subtracted.
  • Corrections due to magnetic mirror effects can be controlled to the required precission during data analysis.

Although the average intensity of the neutron pulse is much lower in a pulsed beam than it is in a continous beam, we still increased the overall number of events in the final analysis by an order of magnitude compared to the last measurement of Perkeo II (Mund et al., 2013). This is the most precise measurement of the beta asymmetry paramter A and the nucleon axial coupling [Märkisch et al., 2019]. The results largely rules out the dark decay interpretation of the neutron lifetime puzzle [Dubbers et al. 2019]. For the first time, best limits on the Fierz interference term b in neutron decay were derived from the data using a simultaneous fit of A and b [Saul et al., 2020]. 

In 2014 and 2015 Perkeo III was installed a third time at the ILL to measure the proton asymmetry parameter C, including its dendence on the proton energy. Again a pulsed, polarized beam of cold neutrons was used. Electrons and protons were detected in the same detector by first converting protons to electrons with the help of ultra-thin and large-size carbon foils. Major parts of the analysis are finished and results will be unblinded once the analysis is completed.

In 2019 and 2020 we successfully performed a measurement of the beta spectrum with the aim to strongly improve the limits on the Fierz interfernce term b using an unpolarised, pulsed neutron beam. A dataset comparable in size to the 2009 data was obtained. Major imrovements are much improved stability, uniformity and resolution of the electron detectors; frequent calibrations and characterisation of the detectors; and drastically improved background levels. Analysis of the data is ongoing.


The successor to the Perkeo instruments is the Proton Electron Radiation Channel (PERC) instrument. It is currently being set-up at the FRM II, Garching by an international collaboration.