Lifetime Measurements Of Excited States In ^137,138 Nd

The lifetime measurement of nuclear excited states is an attractive topic in the experimental studies of nuclear structure physics.The nuclear level lifetime is closely related to the transition probability from this level to other levels.More detailed deeper insights into nuclear structure can be obtained through experimental measurement of level lifetime.It also provides important and fundamental data for the development of the nuclear theories.However,it is a challenge for the measurement technologies due to the shortness of lifetime of nuclear excited states,which is generally in the magnitude of picosecond.There are only a few laboratories in the world where the measurement technology of nuclear lifetime in ps is developed.In our country,China Institute of Atomic Energy?CIAE?has recently developed the technology based on Doppler Shift Attenuation Method?DSAM?and Recoil Distance Doppler Shift?RDDS?for the measurement of lifetime in magnitude of ps.In this work,Recoil Distance Doppler Shift?RDDS?method is employed to measure the average lifetime of nuclear excited states.In offline analysis,the differential decay curve method?DDCM?is applied to process the RDDS data.By using this analysis method,one only needs to consider model-independent direct observables when processing experimental data.As compared with the traditional method,the DDCM can simplify the calculations and lower complex uncertainties,ensuring more reliable results of data analysis.The experiment was performed at the HI-13 tandem accelerator in CIAE.Excited states of ^137,138Nd where populated with the heavy-ion fusion-evaporation reactions¹²³Sb(¹⁹F,xn).The energy of the ¹⁹F beam is 85 MeV.The target consists of a 0.9mg/cm² foil enriched in ¹²³Sb to 97%,and the foil is evaporated onto a backing foil made of 2.2 mg/cm^{2 181}Ta.The target foils were mounted on to a plunger device which is designed and manufactured by CIAE.In addition,a stopper foil,made of¹⁸¹Ta with a large mass thickness?>10 mg/cm²?was mounted behind the ¹²³Sb target and served to stop the recoiled flight nucleus.The ¹⁹F beams ejected from ion source and after collimation in the accelerator tube,they bombarded on the experimental target.They first penetrated the tantalum backing and then reacted with the target nucleus ¹²³Sb,resulting in production of excited states in ^137,138Nd,etc.These nuclei recoiled from the target and then flied towards the thick stopper ¹⁸¹Ta foil in the vacuum chamber in a speed of about 1.15%light speed,and stopped on the stopper foil.The detectors array used in the experiment consisted of 8 high-purity germanium detectors with BGO Compton suppressors and one planar detector.Lifetimes of three energy levels,namely the 2⁺and 4⁺levels in ¹³⁸Nd and the 15/2^-level in the ¹³⁷Nd have obtained in this work.In addition,the corresponding transition probability B?E2?is calculated from the measured energy level lifetimes in ¹³⁸Nd.The B?E2?values are an important experimental data for the structural studies for even-even nuclei.The properties of the first excited 2⁺states in even-even nuclei are a hot topic in nuclear physics.In recent years,B.Pritychenko et al.evaluate systematically B?E2?values for the first 2⁺excited states in even-even nuclei including Nd isotopes.Unfortunately,because the lifetime data in ¹³⁸Nd was still unavailable,the B?E2?value in ¹³⁸Nd could not be given.In this paper,we have obtained the lifetime of the 2⁺and 4⁺states in ¹³⁸Nd on the basis of RDDS,which makes it possible for a more detailed systematic study of B?E2?values in even-even Nd isotopes.