Background subtraction for singles spectra

The gamma-ray counting rate using the ¹⁹Ne beam was 16 times greater than the rate of reaction gamma-rays as estimated using a ¹⁹F beam, and the extra flux was almost all due to 511 keV gamma-rays (cf. figure 1(a)). The observed flux corresponds naively to $\approx 0.02$% of the incident beam, but in fact the beam collimators were shielded and the total intercepted beam was more like 5%. The 0.02% equivalent number is, however, instructive to reflect upon.

 

Figure: Gamma ray energy spectra from the ¹⁹Ne + ⁴⁰Ca reaction: (a) all singles, (b) in coincidence with charged particles, (c) in (slow) coincidence with another gamma ray, and (d) with full background subtraction. Data from ref. [1].

Requiring a charged particle coincidence is quite effective at highlighting reaction gamma-rays, as shown in figure 1(b), and is better than simply demanding a (slow) gamma-gamma coincidence (cf. figure 1(c)). Background subtraction can be performed using the timing relative to the beam, for singles events, resulting in a spectrum which is equal in quality to spectra obtained using stable beams (see figure 1(d)).