Investigation of free volume in polymer by using synchronized irradiation technique of pulsed positron beam and laser

为了利用激光同步系统研究正电子与材料的相互作用，我们研制了基于L波段和S波段电子直线加速器的两种高密度、高计数率的短脉冲正电子束。设计并建造了一台基于L波段直线加速器(最大能量：38 MeV，脉宽：20ps)的新装置，用于用超短脉冲激光激励正电子来研究正电子在材料中的状态。在实验中，通过测量湮没伽马射线，从靶发出的能量对应于外加电压的正电子数约为2500个/脉冲，脉冲宽度约为20 ns(半高宽)。这些值与计算估计值相比是相当低的。这一结果主要归因于强电子像对正电子束的不正确定位。激光照射可引起特殊分子的激发和振动等现象，并通过相互作用影响正电子束的寿命。我们还研制了另一台基于S波段直线加速器的短脉冲正电子束流装置，用于这类实验。这种正电子束的脉冲宽度可达到约370ps。利用这种正电子束，我们开始测量了几种结构规则的材料，如LB膜和几种衍生物的正电子寿命，以研究上述相互作用。激光激发的实验很快就会开始。

To investigate the interactions between positronium and the materials using laser-synchronized system, we have developed two kinds of short pulsed positron beams based on L-band and S-band electron linear accelerators (linacs) which are characterized by high density and high count rate.A new facility based on L-band linac (maximum energy: 38 MeV, pulse width: 20 ps) was designed and constructed for the purpose of the investigation of the status of positronium in the materials by exciting positronium with ultra short pulsed laser. In the experiments, the number of positrons coming from the target with the energy corresponding to the applied voltage was about 2500 per pulse and the pulse width was about 20 ns (FWHM) by measuring annihilation gamma rays. These values were rather low compared with the computational estimation. This results could be mainly ascribed to improper positioning of the positron beam because of bright electron image.The several phenomena such as excitation of the specialized molecules and vibration, can be introduced by laser irradiation and the lifetime of the positronium is influenced through the interactions. We also developed another new short pulsed positron beam facility based on S-band linac for the use of this kind of experiment. The pulse width of this positron beam could be attained about 370 ps. Using this positron beam, we started the measurement of positron lifetime in several materials with regular structure such as LB films and several derivatives, to investigate the interactions mentioned above. The experiments with laser excitation will be started soon.