LEVANTAMIENTO DE INFORMACIÓN DE ACTIVIDADES DE LOS PROCESOS

Energy and time spectra of the signals from the electron paddles are considered here. The 2D-histogram in Fig. 5.6 shows the energy Ee versus the time te of electron signals detected in the electron paddles. The time distribution of the events with energies above the plotted line have an exponential decaying time distribution with a decay constant corresponding approximately to the muon lifetime. Events with energies below that line have a higher level of time–independent background with respect to those above this line. This flat time distribution below the line corresponds to single photo–electron noise generated at the PMT cathode and neutron induced signals in the paddles scintillators and light–guides. In the search of the laser resonance it is important to accumulate all possible statistics. Therefore also signals below the single photo–electron line have been accepted. The acceptance of this low energy signals is motivated by the fact that also a large amount of muon–correlated signals have been detected below this line. Probably this single–photon signals arise from ˘Cerenkov light produced by electrons striking the large Lucite light–guides but not the scintillators. These light–guides mounted down–stream are

5.5 79 0 200 400 600 800 1000 1200 0 2 4 6 8 10 12 Time [µs] Energy [ ADC channel ]

Figure 5.6: 2D spectrum of time te and energyEeof the signals detected in the electron paddles.

Similar result for the other paddle. Single photo–electron signals are expected to be below the solid horizontal line.

transporting the light produced in the paddle scintillators to the PMTs which are placed in a region of low magnetic field. The one–dimensional projection of the above 2D-histogram for energies below the solid line is shown in the top part of Fig. 5.7. As can be inferred from the histograms the number of detected “good” electrons is increased by 35% when the ˘Cerenkov are included. As will be explained later in Appendix H, the inclusion of the noise in the paddle PMTs causes an increase of about 60% of the background events in the laser time window. At our low event rates due to Poisson statistic it is advantageous to increase the rates at a slightly worse signal–to–background ratio.

The time distribution shown in the bottom part of Fig. 5.7 for signals above the solid line — originated by muon decay electrons — should have an exponential time distribution with a lifetime of 2.197 µs (muon lifetime). A deviation from this behavior is observed which has to be attributed to the time–dependent drift of the µp atoms in the hydrogen gas. Two effects lead to the deviation from the expected behavior. First, while the µp atoms move toward the walls of the gas target, the electron efficiency increases due to an increase of solid angle because theµp atom is approaching the detectors. Second, when the

µp atoms impinge on the target walls muon transfer to an atom of the walls occurs. The muon lifetime in such atoms changes because of nuclear capture process (µ−_+p→n+ν

µ) which increases approximately asZ4 for light elements. If the muon is transfered to the C atoms of the polypropylene foils this effect is small, the muon lifetime is _∼2.0 µs instead of _∼2.2 µs [101]. However if transfer occurs to Zn or Se atoms which are the coating material of the cavity mirrors the muon lifetime is reduced to 160 ns. This will cause a time–dependent reduction of the number of measured electrons. At late times when all muons have transferred to the walls, the lifetime should be theµC lifetime since the muons captured in higherZ materials have already disappeared.

This time behavior of the electron detection is shown in Fig. 5.8 using the event class

xe. The distribution of the time difference te−tx is plotted. This distribution is similar to the time distribution of the electrons in the electron paddles. This time spectrum is

80 Measurements 102 103 104 0 2 4 6 8 10

Events

Time [µs]

Events

Figure 5.7: (Top): Time spectrum of electron paddle signals below the line plotted in Fig. 5.6

corresponding to single photo–electrons. The exponential part has to be attributed to ˘Cerenkov

light produced in the light–guides whereas the flat component arises from the single photo–electron noise. (Bottom): Time spectrum of the corresponding signals above the line. The different shape of the two peaks has to be attributed to variations of the detection efficiencies as a function of

time, caused by the drift of the µp atoms in the gas target. Only the xeevent class is considered

in these spectra.

shown since, as will be later described, it can be used also for a timing calibration of our detectors. Moreover it includes contributions of the electron detection in all the electron detectors, LAAPDs and D3 included. As will be explained later various background effects

have a similar time distribution (cf. Chapter H).

It has to be noticed that this spectrum is constructed releasing the definition that a coincidence signal has to be attributed to an electron. Any LAAPD signal with energy below Ehi

x classified here as x ray, while theelectron signal from any other detector may occur simultaneously. The sharp peak at time te−tx = 0 is due to (physical) electrons detected at least in two detectors, one in an LAAPD where it deposits a low energy signal which is considered as an x ray.

As can be seen from Fig. 5.8, the electron detection probability increases at early times (100₋400 ns) which has to be attributed to the drift ofµp atoms to the target walls. At medium times (0.8₋2 µs) the decay time is considerably faster than the muon lifetime. This is caused by the fact that about 30% of the µp atoms drift to the ZnSe surface of the cavity mirrors, where most of the muons undergo nuclear capture without emitting a high–energy electron. The number of electrons from muon decay is correspondingly decreased in this time interval. Only at later times, the measured slope corresponds to theµC lifetime if a flat background is taken into account. This flat background is caused by the paddles noise.

5.6 81 103 104 105 0 1 2 3 4 5 6 7 8

t

- t

[µs]

Events / 25 ns bins

∼e

A e

₊

_C

Figure 5.8: Time spectrum of the time differences te−tx for the xe event class. The time 0

represent the time of x–ray detection. No DELE cut is applied. The sharp peak at time 0 is due to the electrons detected at least in two detectors; once also in an LAAPD, where the electron

deposited an energy below the Ehi

x threshold, and therefore is considered as an x ray. The two

dashed vertical lines represent the DELE cut time window usually applied (not for this histogram)

for the analysis of the data. The data are fitted for times larger than 2 µs with an exponential

function with 2.0µs lifetime (corresponding to theµClifetime) and a flat background from paddles

noise (red functions). The blue curve represents the time spectrum if transfer to the wall would not occur.

A detailed analysis of the time integrated electron detection efficiency for the various detectors has been reported in Ref. [16]. The total probability to detect a muon–decay electron is _∼66% for te −tx ∈ [0.1,7.1 µs]. An exclusive contribution of the electron detectors (paddles+D3) is ∼35%, and for the LAAPDs∼13%. The remaining ∼18% are

detected in both detector types.