Multichannel decay

M.L. Du, A. Dalgamo, Artificial-channel method for multichannel-decay-resonance energies and widths, Phys. Rev. A 43 (1991) 3474. 

In addition, several examples of application were cifed. These include phenomena such as, ac field-induced sfabilizafion of resonances, or long-time NED and ifs relafion fo issues of irreversibilify at the quantum level, or the profile of phofoabsorpfion cross-secfions for resonance fo resonance fran-sifions, or fhe fime-dependenf coherenf excifation and multichannel decay of multiply and inner-hole excifed sfafes, or aspects of the physics of fhe... 

At this point it is important to mention that the experimental setup used for luminescence decay-time measurements is similar to that of Figure 1.8, although the light source must be pulsed (alternatively, a pulsed laser can be used) and the detector must be connected to a time-sensitive system, such as an oscilloscope, a multichannel analyzer, or a boxcar integrator (see Chapter 2). 

Some of the earliest applications of MQDT dealt with vibrational and rotational autoionization in H2 [21-25]. One concept that emerged from these studies is that of complex resonances [26], which are characterized by a broad resonant distribution of photoionization intensity with an associated rather sharp fine structure. These complex resonances cannot be characterized by a single decay width they are the typical result of a multichannel situation where several closed and open channels are mutually coupled. The photoionization spectrum of H2 affords a considerable number of such complex resonances. 

Figure 4. Decay of CF2C102 radicals in the reaction, CF2C102 + N02 =s CF2-C102N02. The peroxy radical was monitored by the signal at m/z = 82 (CF202) +, which was accumulated for 4748 flashes. Each channel of the multichannel scaler represents a time increment of 0.1952 ms.

With single-photon exposure, excitations may decay either through a variety of processes including chain scission and fluorescence (47). We would therefore expect to observe fluorescence from two-photon excitation as well. To observe the fluorescence, we used a Spectra Physics mode locked dye laser system, operating with Rhodamine 560 dye. This was focused onto the polymer film, and the emitted light collected into a spectrometer with a Princeton Instruments Optical Multichannel Analyzer (OMA) attachment. 

Coincidence techniques have also been used for Compton interference reduction in the use of large volume Ge(Li) detectors together with plastic scintillator anticoincidence shields 70), In some cases it might be desirable to use the coincidence electronics to gate the multichannel analyzer to accept only non-coincident pulses. In 14 MeV neutron activation procedures the annihilation radiation resulting from the decay of 13N produced indirectly from the carbon in the plastic irradiation unit may be discriminated against by gating the analyzer to accept only non-coincident events. 

The spectrum of / decay, or the (i spectrum, is a distribution of ejection probabilities for the ji electron versus its kinetic energy. As an initial formula for deriviing an expression for the / spectrum we will use Eq. (8). As was mentioned above, the ft decay in a molecule is a multichannel process owing to the fact that both the parent and the daughter molecules may be in different electronic, vibrational, and rotational states. Thus, Eq. (8) should be employed for each channel of the reaction. Using the energy conservation law and the factorization of the matrix element, Eq. (12), we obtain the following expression for the probability of ft decay in the channel 0 n ... 

Samples (156) were taken from 54 reference lithic pieces that represented five rock types. These samples were analyzed at the SLOWPOKE Reactor Facility of the University of Toronto. They were irradiated for 1 min at 2 kW, or for 1 or 2 min at 5 kW (depending on their radioactivity level in preliminary tests). Upon removal from the reactor, the samples, which weighed between 0.1 and 0.3 g, were left to decay for 18 min and were counted for 5 min with a Ge(Li) y-ray detector coupled to a multichannel analyzer. Trace element concentrations were calculated with the comparator method (7). The 15 elements examined were barium, titanium, sodium, aluminum, potassium, manganese, calcium, uranium, dysprosium, strontium, bromine, vanadium, chlorine, magnesium, and silicon. The first seven of these elements were the most useful in the differentiation of major rock types. 

The voltage pulse produced by the TAC is fed to the multichannel analyzer (MCA), and is stored in a specific channel according to its amplitude, and hence time, post-excitation. The probability of a single photon event being counted is high soon after excitation and decreases with time. Repetitive operation of the TAC produces a probability histogram for the detection of fluorescence photons, which is identical to the fluorescence decay curve. 

---