エネルギー分散型X線分光

energy-dispersive X-ray spectroscopy, EDS

試料から発生した特性X線を直接半導体検出器で検出し、電気信号に変えて分光分析する手法。検出した特性X線のエネルギーに比例したパルス電流を生じさせ、これを多チャンネル波高分析器で選別して測定する。波長分散型と比べ軽い元素(B: ボロン以下)は分析できないが、X線の検出効率は高い。照射電流量は波長分散型より少なくてすむので(数pA～数nA)試料へのダメージは少ない。通常の分解能は～140eV（Mn: マンガンのKα発光(5.9keV)に対して）程度である。統計誤差で決まる分解能は発生X線のエネルギーEの平方根×√３程度である（生成される電子数nは、バンドギャップエネルギーを～3eVとして、n～E/3、統計誤差Δn～√n。したがって、エネルギー巾（誤差）～Δn・3＝√E・√3）。最近はBe(ベリリウム)も分解できる検出器も開発されている。定量精度は0.5～5％である。EDSはEPMA（分光結晶を用いる）に比べて、空間分解能は2桁高いが分析の定量精度は1桁悪い。略称はEDSであるが、EDXともいう。

Energy-dispersive X-ray spectroscopy (EDS) is an element analysis method. Characteristic X-rays generated from a specimen are detected by a semiconductor detector and converted into electric signals. In the EDS analyzer, the pulse currents that are proportional to the energies of the detected characteristic X-rays are generated, and then these currents are measured with a multi-channel pulse-height analyzer. EDS has higher detection efficiency of X-rays than WDS, but the analyzing power of light elements is lower than WDS. (EDS cannot analyze elements from boron (B) on down.) Since the illumination current of the electron beam for EDS can be decreased from several pA to several nA compared with WDS, the beam damage to a specimen is small. Normally, the resolution of EDS is ～140 eV for Mn Kα emission at 5.9 keV. The resolution determined by statistical counting error is around the square root of energy E of the generated X-ray × √3. (The number of created electrons n is obtained as n～E/3 by taking the band gap energy as ～3 eV. Since the statistical error Δn is ～√n, the energy spread (error) is obtained to be ～Δn・3 = √E・√3.) Recently, an EDS detector that can resolve a beryllium (Be) peak has been developed. Its quantification accuracy is 0.5 to 5%. Compared with EPMA that uses analyzing crystals, EDS provides high spatial resolution, 100 times better than EPMA but shows 10 times worse quantification accuracy than EPMA. "EDX" is also used as the abbreviation of energy-dispersive X-ray spectroscopy.