Single-Electron Detection and Spectroscopy via Relativistic Cyclotron Radiation

It has been understood since 1897 that accelerating charges must emit electromagnetic radiation. Although first derived in 1904, cyclotron radiation from a single electron orbiting in a magnetic field has never been observed directly. We demonstrate single-electron detection in a novel radio-frequency spectrometer. The relativistic shift in the cyclotron frequency permits a precise electron energy measurement. Precise beta electron spectroscopy from gaseous radiation sources is a key technique in modern efforts to measure the neutrino mass via the tritium decay end point, and this work demonstrates a fundamentally new approach to precision beta spectroscopy for future neutrino mass experiments.

Figure 1

A picture of the waveguide insert (left), where both the gas lines and trapping cell can be seen, and a corresponding schematic (right) of the receiver chain, consisting of cascaded cryogenic amplifiers, a high-frequency stage, and a low-frequency stage. Calibrated radio-frequency signals can be injected into the cryogenic receiver and through the WR42 waveguide section. The high-frequency band of interest, with a width of 2 GHz centered at 26 GHz, is then mixed down to be centered at 1.8 GHz with the same 2-GHz bandwidth. A second amplification and mixing stage is used to further amplify the signal and shift the center frequency. Data can be recorded either using an 8-bit digitizer or through a real-time spectrum analyzer (RSA).

Figure 2

A typical signal from the decay of ${}^{83\mathrm{m}}\mathrm{Kr}$ characterized by an abrupt onset of narrow-band power over the thermal noise of the system. The measured frequency reflects the kinetic energy of the electron, in this case 30 keV. The frequency increases slowly as the electron loses energy by emission of cyclotron radiation, ending in the first of six or possibly seven visible frequency jumps before the electron is ejected from the trap. The frequency-time window shown represents only a portion of an extended event lasting more than 15 ms. The sudden jumps result from the energy loss and pitch-angle changes caused by collisions with the residual gas, predominantly hydrogen. The most probable size of the energy jump, as determined from many events, is 14 eV.

Figure 3

The kinetic energy distribution of conversion electrons from ${}^{83\mathrm{m}}\mathrm{Kr}$ as determined by CRES for a trapping current of 800 mA. The spectrum shows the 17-, 32-, and 30-keV complex conversion electron lines. The shaded region indicates the bandwidth where no data were collected. Inset: With the trap current reduced to 400 mA, the feature at 30.4 keV is resolved into 3 lines. Also visible as low-energy shoulders on these lines are shakeup satellites.