Lifetime spectrometers

Fig. 1.4. (a) Schematic of a positron lifetime spectrometer the star indi-... 

Fluorescence lifetimes were measured with a Laser Strobe Fluorescence Lifetime Spectrometer (Photon Technology International) with 337 nm laser pulses... 

K. Maier and R. Myllyla Positron Lifetime Spectrometer with (fy-Coincidences, in Positron Annihilation, eds. R. H. Hasiguti and K. Fujiwara (Japan Institute of Metals, Sendai, 1979) p. 829. 

Figure 2.24 Decay curve acquired with a PTI TimeMaster TM-3/2003 Ni -dye laser fluorescence lifetime spectrometer. Curtosey from Photon Technology International.

The intensities of classic lifetime experiments are well within the TCSPC range. Furthermore, sensitivity, time resolution, and accuracy are often more important than short acquisition time. Therefore many classic lifetime systems still use the classic NIM-based TCSPC technique. The general principles of fluorescence lifetime experiments are described in [308, 389], and various fluorescence lifetime spectrometers are commercially available. 

The t q)ieal optical setup of a fluorescence-lifetime spectrometer is shown in Fig. 5.5. The sample is exeited by a laser. The excitation intensity is adjusted by a variable neutral density filter. An additional bandpass filter may be required to bloek unwanted emission wavelengths of the laser. 

R.W. Wijnaendts van Resandt, R.H. Vogel, S.W. Provencher, Double beam fluorescence lifetime spectrometer with subnanosecond resolution Application to aqueous tryptophan, Rev. Sci. Instrum. 53, 1392-1397 (1982)... 

The excitation wavelength selector can be either a filter or a monochromator. Filters offer better detection Hmits, but do not provide spectral scanning capabilities. Often, a filter is used in the excitation beam along with a monochromator in the emission beam to allow emission spectra to be acquired. FuU emission and excitation spectral information can be acquired only if monochromators are used in both the excitation and emission beams. In modern instruments with array detectors, a polychromator is used in the emission beam instead of a monochromator. Recent research instraments are able to scan both wavelengths automatically and combine all data into a 2D excitation—emission spectrum. In lifetime spectrometers, a pulsed light source and a gated detector are synchronized in order to measure the time dependence of the luminescence emission. 

Saito, H., Nagashima, Y, Kurihara, T., and Hyodo, T., A new positron lifetime spectrometer using a fast digital oscilloscope and Bap2 scintillators, Nucl. Instrum. Methods Phys. Res., A, 487, 612-617 (2002). 

This work investigated PIM-1 membranes in the three states discussed above. Nickel-foil supported NaCl was used as a positron source and stacks of film samples, each about 1mm thick, were placed either side of the source. Annihilation lifetime decay curves were measured with an EG G Ortec fast-fast lifetime spectrometer. Measurements were made both in air and under an inert atmosphere (N2). However, o-Ps lifetimes in air were reduced due to quenching by oxygen, so only results obtained under N2 are discussed here. Results were analysed in terms of a four component lifetime distribution, which allowed obtaining better statistical fit. The two longest lifetimes, T3 and T4, for PIM-1 in... 

The case is even worse with lifetime spectroscopy. The moderation process erases the information on the birth of positrons thus, performing lifetime spectroscopy is almost impossible. Although this problem was resolved by the construction of pulsed beams (Mills 1980), the time resolution of these systems has only reached that of a normal lifetime spectrometer recently (Hamada et al. 2000 Bauer-Kugelmann et al. 2001). 

Boxcar averagers and their use in signal recovery are described in (39). PTI (Photon Technology International) manu ctures a series of fluorescence lifetime spectrometers (models C-70 to C-73) that use a stroboscopic detection technique (i.e. a gated PMT, as described in the previous paragraph) and either nanosecond fiashlamp or nitrogen/ e laser excitation. 

The t5q)ical resolution of the lifetime spectrometers is 200-250 ps, FWHM. This value is of the order of magnitude of the positron lifetimes in solids. Therefore the determination of the lifetime components r, is a delicate operation. Various methods are available (1) least-squares fit of a model spectrum convoluted with the resolution function of the spectrometer (Kirkegaard and Eldrup 1974) (2) Laplace transform (Gregory and Zhu 1990) and (3) maximum entropy (Shukla et al. 1993,1997). The last two methods provide... 

The PALS spectra were collected using a standard fast-slow lifetime spectrometer. The air was evacuated from the measurements chamber to p 0.3 Pa in order to avoid oxygen influence on o-Ps lifetime. Positronium lifetime measurements (PALS method) are based on the relation between ortho-positronium (o-Ps) lifetime and the size of fi volume, in which o-Ps is trapped. PALS spectra were processed as described in Ref. [10]. 

Figure 27.3 (a) Block diagram of positron lifetime spectrometer (b) Typical lifetime spectrum of... 

The PAL measurements were performed using an ORTEC standard fast-fast coincidence circuit of a positron lifetime spectrometer with time resolution of 230 psec (full width at half maximum (FWHM) of the prompt coincidence curve). Integral statistics for each PAL spectrum included at least 10 counts. A high purity and defect free single crystal Silicon sample was used as a reference. Its PAL spectrum consisted of a single component with a lifetime of 220 psec. Measurements were performed either at ambient conditions, i.e. in contact with air, or in nitrogen at 1 atm in a special cell. 

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