DELAYED COINCIDENCE METHOD FOR PICOSECOND LIFETIME MEASUREMENTS

Transcription

1 306 DELAYED COINCIDENCE METHOD FOR PICOSECOND LIFETIME MEASUREMENTS ZHANG WEIJIE China Institute of Atomic Energy The advanced time delay (ATD) technique, based by delayed coincidence method, and other derived techniques are widely used for short lifetime measurements. This paper presents the principle of the ATD technique, the crucial factors which affect the timing performance of a fast timing setup, and the optimizations of them. 1. Introduction The lifetime of a nuclear excited state is one of the most important parameters for nuclear structure studies. Different methods for lifetime measurement, cover a wide range of nuclear lifetimes ( s), have been developed in the past century. The delayed coincidence method, also called fast timing technique, which was firstly introduced in the 1950s, is one of the most widely used methods for lifetime measurement. As a result of the development of new ultrafast scintillators, the sensitivity of the fast timing technique was improved. In 1989 H.Mach and his colleagues established an advanced β-γ-γ coincidence method, often called advanced time delay (ATD) method, to measure the lifetimes of neutron-rich nuclei. In this method all-sided optimizations were introduced to improve the time resolution down to the low-picosecond range. The principle of the ATD method and the timing optimizations had a profound influence on the progress of delayed coincidence techniques for lifetime measurement. 2. The Advanced Time Delay (ATD) Technique The ATD technique was designed for neutron rich nuclei, whose production and separation methods made the Doppler techniques difficult to be adapted. Neutron-rich nuclei generally decay to levels with excitation energy of 2-3 MeV by strong β-feeding. By a typical β-decay, a low-spin

2 307 isomer generally populates a number of excited levels which de-excite to the ground state by a cascade involving some γ-transitions. For this case the β-γ-γ coincidence method is suitable. On the other hand, β-decay of a high-spin isomer typically populates a long sequence of γ-transitions. For this case γ-γ-γ coincidence is preferable. Fig. 1. General feature of decay schemes following β-decay of low-spin(a) and highspin(b) isomers. In H.Mach s ATD method, the lifetime of an excited level is obtained by using a β-γ delayed coincidence to measuring the time difference between the β radiation feeding the level and the γ radiation decaying it. The β particles are detected by a thin NE111A plastic scintillator and the γ rays are detected by a BaF 2 crystal, the fastest inorganic scintillator so far. However, because of the poor energy resolution of BaF 2 crystal, a HPGe detector with top energy resolution is necessary to be employed to selected the desired decay brunch. Fig. 2. The principle of the fast timing method for measuring the lifetime of the excited state labeled with.

3 Time Resolution Time resolution is the parameter directly indicates the lowest time difference a coincidence setup be able to determine. It is generally described with the FWHM of real prompt time spectrum, which is Gaussian distribution, obtained by measuring two simultaneous events using the setup. The total time resolution of a setup is contributed by both two timing brunches. Each of them can be formulated as, δt = F W HM 2 σ 2 Sc + σ2 P MT + σ2 T P Where σ Sc, σ P MT, σ T P are the time differences introduced by the scintillator, the photomultiplier, and the time pickoff respectively. Each one should be decreased in order to optimize the time resolution Scintillators One of the most important contributions for the time resolution of a fast timing setup is the intrinsic timing property of the scintillator. The time jitter of scintillator can be described as, τf σ Sc η = k N f where η called figure of merit, τ f and N f are the decay time and the light yield of a scintillator respectively Intrinsic Time Properties Fig. 3. Properties of some types of inorganic scintillator.baf 2 is the fastest inorganic scintillator so far.labr 3 has a time resolution comparable to that of BaF 2 and the best energy resolution in commonly used scintillators.the properties of LaBr 3 depends on its cerium doping.

4 309 As shown in the chart, BaF 2 has ultra-short decay time and very small η of 0.6, so that is the fastest inorganic scintillator so far, and was employed by H.Mach into his ATD method as γ timing detector. On the other hand, LaBr 3 presents not only a good time resolution that comparable to that of BaF 2, but also a good energy resolution down to about 3%, which helps it available for selecting the γ radiation decaying the interesting level in lifetime measurement. Therefore, LaBr 3, instead of BaF 2, are the most preferable γ timing detector for fast timing measurement since it was invented in The energy spectrum of β-decay is a continuum since the Fig. 4. Properties of some types of Pilot U and NE111A. decay energy is shared between the electron and the neutrino. For β timing detector, as a result, good time resolution is the only required parameter. NE111A and Pilot U are both characterized by so that widely used in β timing detection Geometry The geometry of a scintillator affects its timing performance as well. In order to optimize the time resolution of the γ timing detectors, BaF 2 of LaBr 3 crystals prefer to manufactured into shape of truncated cone, which reduces the time spread associate with the light collection process. Otherwise, for giving better β timing in β-γ coincidence, plastic scintillators are commonly manufactured into thin sheets with few-millimeter thickness for the purpose of uniform the timing respond for β particles in different energy Photomultipliers Tubes (PMTs) A scintillator conventionlly be used by coupling to a photomultiplier tube (PMT), which converts the emitted photons into an electronic signal. The time difference of PMT can be considerably reduced for increased photocathode quantum efficiency, increased secondary emission yield and reduced

5 310 total transit time. The PMTs should be selected which spectral responses fit with the maximum wave length of the scintillator and have a large quantum efficiency, have short rise time, short transit time and small transit timespread. For the NE111A(or Pilot U), BaF 2 and LaBr 3, the most preferable PMTs are XP2020, XP2020Q and XP20D0, respectively. All of these three types are characterized by short transit time and small transit time-spread. The voltage divider and signal out mode also influence the time respond of a PMT. For XP2020 and XP2020Q, using dynode signal for timing can improve the time resolution in comparison to the traditional anode mode Constant Fraction Discrimination (CFD) There are three important sources of error in time-pickoff measurements: walk, drift, and jitter. Walk introduced by the uncertainty of pulse shape and amplitude; while drift relates to the aging and temperature variations;and jitter caused by noise and by statistical fluctuations of the signals from the detector. These can be considerably eliminated by using Constant Fraction Discrimination (CFD). In a CFD, the input signal is delayed and Fig. 5. Signal formation in a constant-fraction discriminator. a fraction of the undelayed input is subtracted from it to produce a bipolar pulse. The zero crossing is detected and used to produce an output logic pulse. The CFD timing mode significantly reduces the timing difference σ T P introduced by the time-pickoff and then benefits for the precision of fast timing measurements. 4. The Centroid Shift Method The most straightforward way to determine the lifetime is to use the slope method. In the delayed time distribution in a semi-logarithmic plot, the slope of the straight line of decay is directly the decay constant λ, the reciprocal of the lifetime τ. This method is available for lifetimes that longer

6 311 than the FWHM of the prompt coincidence curve.it means that,the lifetimes can be determined by using slope method with an all-sided optimized fast timing setup, cannot shorter than several hundreds of picoseconds. For the picosecond lifetimes, centroid shift method should be used. The first moment of the delayed time distribution C(D) is defined as tc (D) dt C (D) = C (D) dt The same as the centroid of its corresponding prompt coincidence curve C(P), tc (P ) dt C (P ) = C (P ) dt Then the lifetime can be determined by the difference between C(D) and C(P), τ = C(D) C(P ) The centroid shift method is widely used with the ATD technique for measurements of few-picosecond lifetimes. 5. Status and Prospects Since the ATD technique was introduced by H.Mach in 1989, numbers of level lifetimes of different nuclei have been measured with picosecond precision by ATD technique or other derived fast timing methods. Delayed coincidence method practiced a renaissance since the invention of LaBr 3, and will continually progress in the future with the appearances of new more sensitive detectors and new analysis techniques. References 1. H Mach, R L Gill and M Moszynski,Nucl. Inst. and Method. A (1989). 2. M Moszynski and H Mach,Nucl. Inst. and Method. A (1989). 3. Jan Jolie,Alfred Dewald and Patrick H. Regan,Fast Timing with LaBr3(Ce) Scintillators and the Mirror Symmetric Centroid Difference Method I Deloncle, B Roussiere, M A Cardona et al,j. Phys (2010). 5. Dan Gabriel GHITA, U.P.B. Sci. Bull. Series A. Vol.70. Iss.4(2008). 6. T.J.Paulos, IEEE Transactions on Nuclear Science. Yol. NS-32, No.3, June ORTEC,Principles and Applications of Timing Spectroscopy.