Subject Number :S-17-MS-2029
Support Type :Common use (including technical support necessary for the training),
Proposal Title (English) :Development of a Cryo-Cooling System for Scanning Transmission X-ray Microscopy
Username (English) : T. Ohigashi1, 2, F. Kaneko3, Y. Inagaki1, T. Yano1, H. Kishimoto3 and N. Kosugi1, 2
Affiliation (English) : 1UVSOR Synchrotron, Institute for Molecular Science, Okazaki 444-8585, Japan
2School of Physical Sciences, SOKENDAI (The Graduate University for Advanced Studies), Okazaki 444-8585, Japan
3Sumitomo Rubber Industries, Kobe, 651-0072, Japan

A scanning transmission X-ray Microscope (STXM) is a powerful tool to obtain 2-dimensional chemical states with high spatial resolution. By using the STXM in the soft X-ray region, core-level absorption edges of light elements, carbon, nitrogen and oxygen, are available for the measurement. Generally speaking, the radiation damage by X-rays is lower than that by electron beams [1]. This means that even organic materials, such as polymers, carbon nanotubes and biological specimens that are very sensitive to radiations, can be targets of the STXM. However, in spite of the lower radiation damage of X-rays, some samples show chemical or morphological changes during the measurement. For example, rubber, one of reasonable targets for the STXM analysis [2, 3], is easily damaged by the irradiation of X-rays. It is known that the cooling of rubber samples reduces radiation effects [4]. In the present work, therefore, we have developed a cryo-cooling system.
A Dewar vessel is placed on the STXM. Liquid nitrogen in the vessel cool a sample mounting plate through copper braids. A sample holder is equipped with two sets of a heater and a temperature sensor. One of these combinations is for the temperature control of samples and the other one is for keeping the temperature of stages constant. These heaters are operated by PID controllers. Main issues of the cryo-cooling system for STXM are the vibration generated by bubbling of liquid nitrogen and the thermal stability. In the present work, by optimizing thermal insulation and stiffness of the copper braids, the vibration at the sample is successfully reduced down to 50 nm (peak to valley). By using this system, the sample mounting plate can be cooled down to -90C.
A thin specimen (thickness of 250 nm) of vulcanized rubber was used as a sample. After measuring an energy stack at nitrogen K-edge by changing dwell time as 1, 2 and 4 ms, the energy stack at carbon K-edge was measured to evaluate damages on the sample. NEXAFS spectra around carbon K-edge at RT and at cryo contion of -85C and an STXM image of a damaged sample are shown in Fig. 1. In Fig. 1(a) and 1(b), main peaks are 285.4 and 288.0 eV, corresponded to C=C * and C-H * transition respectively. The damage by X-ray irradiation mainly appears on 285.4 and 288.0 eV as decrease of intensity and as broadening of peak width. Especially, the peaks at 288.0 eV show apparent difference between RT and cryo conditions. In RT condition, peaks at 288.0 eV are dull and low rather than those in cryo condition although the lowest dose by the dwell time of 1 ms. Furthermore, height of the peaks at 285.4 in RT are lower than those in cryo condition. These differences show that using the cryo-cooling system is effective to reduce the damage on the sample. Currently, critical dose to analyze the vulcanized rubber and improvement of cryo temperature are under discussion.

Fig. 1. NEXAFS spectra of pristine and damaged vulcanized rubber around carbon K-edge (a) at RT and (b) at -85C. An inset in (a) is an STXM image of the damaged vulcanized rubber acquired at 285.4 eV.

[1] H. Ade et al., Science 258 (1992) 972-975.
[2] D. A. Winesett et al., Rubber. Chem. Technol. 76 (2003) 803-811.
[3] D. A. Winesett et al., Rubber. Chem. Technol. 80 (2007) 14-23.
[4] G. Schneider, Ultramicroscopy, 75 (1998) 85-104.