Delayed coincidence method

Delayed coincidence method used to determine radiative lifetime of F(2p 3p)... 

The half-life of the 82-3-keV excited state as determined by two delayed coincidence methods and from the Mossbauer linewidth is = 3-8(3) ns... 

Lifetimes of more than 10" sec may be deter-mined by the delayed coincidence method (Bell I 1 2T ) if there is a preceding radiation. 

It is interesting to compare the reduced transition probabilities obtained by Coulomb excitation with those obtained by the delayed coincidence methods. 

Table 7. Comparison of reduced transition probabilities in even-even nuclei obtained by Coulomb excitation (Huus, Bjerregaard and Elbek) and by the delayed coincidence method (Sunyar).

The repetition rate / of the excitation pulses is chosen as high as possible since the measuring time for a given signal-to-noise-ratio is proportional to 1//. An upper limit for / is determined by the fact that the time T = /f between two successive laser pulses should be at least three times the lifetime r of the measured level A ). This technique is therefore ideally suited for excitation with mode-locked or cavity-dumped lasers. There is, however, an electronic bottleneck the input pulse rate of a TAC is limited by its dead time tq and should be smaller than 1 /td. It is therefore advantageous to invert the functions of the start and stop pulses. The fluorescence pulses (which have a much smaller rate than the excitation pulses) now act as start pulses and the next laser pulse stops the TAC. This means that the time (T — t) is measured instead of t. Since the time T between successive pulses of a mode-locked laser is very stable and can be accurately determined from the mode-locking frequency / = 1 / T, the time interval between successive pulses can be used for time calibration of the detection system [656]. In Fig. 6.91 the whole detection system is shown together with a decay curve of an excited level of the Na2 molecule, measured over a period of 10 min. More information about the delayed-coincidence method can be found in [798]. 

Positron lifetime measurements were carried out by the usual delayed coincidence method (25), The resolution the system, as measured by the prompt time distribution of Co source and without changing the 1.27- and 0.511-MeV bias, was found to be 0.390 ns fwhm. Corrections for the source component, which had an intensity of less than 4%, were made in the usual way by using conventional computational methods. 

More information about the delayed-coincidence method can be found in [11.101]. 

We have shown in previous chapters that the -values of spectral lines are important fundamental data which must be known before detailed calculations of the behaviour of gas discharges, plasmas, or stellar atmospheres can be undertaken. Since it is difficult, in many cases, to make theoretical calculations of f-values to an accuracy of better than 20 per cent, experimental measurements of these quantities are essential. A considerable number of different techniques have been developed for this purpose, many of them involving the determination of radiative lifetimes. In this chapter we discuss two such techniques, namely the beam-foil and the delayed-coincidence methods. In Chapter 8 we shall discuss the determination of the f-values of resonance lines by studies of the profiles of spectral lines and in Chapters 15 and 16 the use of the Hanle effect and optical double resonance methods. 

We discuss first the beam-foil method for the measurement of lifetimes since conceptually and experimentally it is very simple. The technique is usually applied to the measurement of the lifetimes of excited states of atomic and molecular ions. The delayed-coincidence method which we treat in the second half of this chapter is experimentally more complex but it is perhaps the most accurate and widely applicable of the modern techniques. 

The second method uses pulsed lasers and the laser-induced fluorescence is detected by telescope. If the telescope and the laser source have a definite base distance, the crossing of laser beam and the acceptance angle of the telescope define the height of the atmospheric layer at which fluorescence is detected. There is also the technique of delayed coincidence, where the time interval between laser pulse and detected fluorescence pulse determines the distance of the observed molecules from the observer (Lidar)... 

A more direct method for lifetime measurements is the delayed coincidence technique [6] in which the time between an initiation event and the emission of a decay product is measured. A schematic diagram of an apparatus used for the measurement of atomic lifetimes is shown in figure BLIP.5. The slope of the graph of the natural log of the number of decay events as a function of time delay gives the lifetime directly. The... 

In the method of delayed coincidences, the coincidence counting rate between detectors responding selectively to genetically related particles, e.g., a jS- or a-par-ticle and a y-ray or an electron or between y-rays, is plotted as a function of a time delay inserted in the electronic circuits used to detect one of them. The development of this method and of improved electronic techniques has led to the use of resolution times of between 10" and 10 sec. We shall not discuss these methods here, for they have been very adequately reported by Bell [i9], 2ff]. Further analysis of the experimental data has made it possible to measure lifetimes of the order of 10 sec. The limit to the method seems to be determined by the scintillating properties of the phosphor and its physical size. 

The first method, which was that used by Moon, who first detected the effect, made use of a Au source electroplated on to the periphery of a mechanical rotor. The details of both first and second methods have been summarized by Malmfors [i9]. The lifetime of the state can be calculated from the measured cross section for the process, the y-ray energy, and from the Doppler effect. It is instructive to compare the lifetimes obtained by different methods. The lifetime of the 411 kev state of Hg found by Graham and Bell by the method of delayed coincidences was (1.0 0.7) X10" sec that found by Moon and Davey by mechanical motion was (2.2 0.5) X10" sec by Malmfors (by heating), 3.5 X 10 sec and by Metzger and Todd , (2.2 0.2) X10 sec. In the same way, the lifetime of the 209 kev state of Hg found by Metzger and Todd ,... 

The lifetimes of F2 transitions in nuclei of intermediate atomic weight have usually proved too short to measure by the method of delayed coincidences. Of nine nuclei between Mg and Te, Coleman has succeeded in measuring the lifetime of only the first excited state of Te. The upper limits of the lifetimes of the rest are of the order of 10" sec. In heavier nuclei, the energies of the first... 

As in the previous method the delayed-coincidence technique also uses short laser pulses for the excitation of selected levels. However, here the pulse energy is kept so low that the detection probability Pd of a fluorescence photon per laser excitation pulse remains small (Pd < 0.1). If is the probability of detecting a fluo-... 

In olden times the lifetime distribution of short-lived nuclides was measured without a multichannel analyzer using the method of delayed coincidences. In this case, the start signals were delayed and the coincidence rate was plotted against the time of delay. The resulted graph showed the characteristic features of the exponential law, fi om which the decay constant could be determined by calculating the slope of the semilogarithmic (semilog) plot. 

CONTROLLED RADIOACTIVE AND EXCITED NUCLEI RADIATIVE GAMMA DECAY BY THE DELAYED GAMMA-GAMMA COINCIDENCE METHOD... 

Experimental layout for detecting of the effect of controlling the time of radiation nuclear decay based on the delayed gamma-gamma coincidence method for cases thin resonant Fe screen is situated near the controlled radioactive source (a) thin Fe resonant and thick Pb nonresonant screens are situated near the controlled radioactive source (b). 

Similar experiments were conducted in a last time [ 14,15] on the basis of Mossbauers sources ( "Sn02 oxide or "Sn metal) and nearest (not distant) and distributed in space resonance screen, made of Sn02 oxide. Activity of all samples was Q = 1.5 x lO Ci. Powder source (" " Sn02 oxide) and powder absorber (" Sn02 oxide) have been mixed and placed in small plastic scintillator. Analysis of low of gamma decay was conducted by delayed (e — y) coincidence method. 

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