Basics of Spherical Detectors

Schema of a SPC cross-section.The detector consists of a spherical metallic vessel and a small metallic ball located at the center of the vessel. The ball is held in the center of the sphere by an insulated rod, set at a high voltage. The field, varying as 1/ r 2, is highly inhomogeneous along the radius. This allows the ionization of atoms when particles interact, causing the electrons to drift to the central sensor in low-field regions constituting most of the volume, while they trigger an avalanche within few mm around the sensor. The amplification capability combined with the very low capacitance of the sensor allows the sub-keV threshold to be easily reached, and in particular settings, single ionization electron sensitivity. It should be noted that the threshold does not depend on the size of the vessel, anticipating the possibility to handle rather large mass of targets read by a single channel.

Two types of sensors were studied: one with an “umbrella”, mounted on the rod a few cm from the spherical sensor, and set to an intermediate high voltage; the other experiment was done with no “umbrella”. The umbrella was added as a correction to the inhomogenous field produced by the rod; shapes and sizes of the umbrellas are still under investigation to obtain the most consistent electric field.

Set-ups were tuned from electrostatic simulations to obtain the best homogeneity of the field inside the vessel. The homogeneity of response is ultimately assessed by the symmetry of the peak obtained with monoenergetic photons – alphas – from radioactive sources, converted homogeneously inside the volume of gas. The signal is extracted from the HV wire through a capacitor, amplified by a low noise charge amplifier, with time constants ranging from 30 ms to 300 ms. The signal is digitized at a frequency around 1 MHz, and sent to a computer which performs a software trigger after adequate noise filtering to obtain the lowest threshold amplitude.

Getting a low threshold for large mass, however, requires keeping sizable gains at high pressure, which leads to the choice of a small diameter for the sensor. However, the field, which is roughly proportional to the diameter of the sensor, becomes very low at large radii and may not be strong enough to drift electrons to the sensor. Studies on the sensor are being pursued to decouple these two requirements.

Discrimination Capabilities

 

Illustration of the particle identification based on pulse shape discrimination.

 

Tracks: The detector is able to discriminate “tracks” from point-like energy deposition by analysis of the pulse shape (obtained through the voltage amplifier). The shape of the pulse keeps track of the arrival time of the electrons; depending on the gas and HV settings, drift times from electrons originating from the internal surface of the vessel range from 30 to 500 µs.

Point-Like: For point-like energy deposition, the rise time of the pulse is directly linked to the longitudinal diffusion of the ionization electrons drifting through the gas (that is, to the radius of emission), allowing fiducialisation of the events.

Low Pressure: Electron/nuclear recoil discrimination is possible when the detector is operated at low enough pressures to give electrons a sizeable track, while nuclear recoil gives a point-like energy deposition. Efficiencies vary with operating conditions.