Spikes

The most prolific source of background noise is represented by the spikes in the ratemeters produced by passages of high-energy charged particles through the detector slabs. These excess counts in the 1 s ratemeters involve single bins at a time and mostly affect the GRBM band: in fact, the poissonian component of the background noise in the AC counts is the most relevant, as already shown in the previous paragraph (see table ). Therefore, two main features characterize them: the spectral softness, of key importance for distinguishing them from GRBs, and the HTR temporal profile, shaped like a fast rise followed by an exponential decay (fig. ).

Figure: Example of a strong spike occurred in unit 1, GRBM band, at 11:25:43.06 UT, on August 26, 2001. Left panel: 1 s bin profile; right panel: same profile with a 7.8125 ms resolution. The typical exponential decay is clearly visible: in this case, a least square fit estimates the exponential decay constant $\tau = 81 \pm 1$ ms, $\chi^2_r = 1.38$. The dip, visible in the middle of the HTR pulse, is fake and it is due to the 6 bit counters' recycling.

The explanation of the fact that a single particle is responsible for several $10^3$ counts within one second is the following: these particles are often cosmic rays with a high atomic number Z, that excite metastable states of the CsI(Na) crystal of the crossed detector: the time required for these states to de-excite is a fraction of 1 s (up to few 100 ms), which is long when compared to the response times of the on-board electronics: therefore, the phosphorescence light produced in the decay is processed and converted into several counts ([Emigh and Megill, 1954,Hurley, 1978]).