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研究生:黃俊發
研究生(外文):Huang, Jun Fa
論文名稱:Betagauge效能評估
論文名稱(外文):Evaluation of Beta gauge Attenuation Method
指導教授:陳志傑陳志傑引用關係
指導教授(外文):C. C. Chen
學位類別:碩士
校院名稱:國立臺灣大學
系所名稱:職業醫學與工業衛生研究所
學門:醫藥衛生學門
學類:公共衛生學類
論文出版年:2000
畢業學年度:88
語文別:中文
論文頁數:59
中文關鍵詞:Beta gauge
外文關鍵詞:beta gaugeMass Attenuation
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Beta gauge 運作原理為利用貝他射線穿越微粒時射線強度遞減現象,測量濾紙上微粒沈積量,再推算大氣氣懸微粒質量濃度。而目前已知Beta gauge干擾因子為(1)微粒粒徑、(2)濾材均勻度、(3)沈積均勻度、(4)微粒成分、(5)微粒吸濕性。本研究首要目的在於找出對Beta gauge量測結果有所影響之環境因子、或本身設計缺失,並將之干擾定量化,作為往後Beta gauge改良之依據。
實驗將以自製的Beta gauge在低流速的封閉腔中進行,利用微粒產生器產生測試微粒,輔以Po210放射源使氣膠達成電荷中和。使用Beta gauge收集微粒於濾紙上,以輻射偵測器紀錄貝他射線穿過濾紙後強度。同時間以APS(Aerodynamic Particle Sizer)、SMPS(Scanning Mobility Particle Sizer)監測微粒粒徑分佈,TEOM(Tapered Element Oscillating Micro Balance)與濾紙收集法監測微粒質量濃度。實驗項目包括微粒成分、微粒粒徑、環境濕度、濾紙定位、沈積厚度、沈積均勻度、採樣效率、粒徑分徑器測試等。另外將進行戶外採樣,與TEOM、Dust-Trak氣膠偵測器實際比較、瞭解實際環境對於採樣器之影響。
隨著微粒沈積,貝他射線強度亦跟著指數性遞減。在表面質量沈積密度達0.8 mg/cm2,沈積密度x與貝他射線強度比值I/I0(沈積後強度/沈積前強度)仍呈現良好之指數關係。實驗中採用酒石酸鉀鈉(PST)、食鹽(NaCl)、硝酸鉛(Pb(NO3)2)為測試微粒,微粒計數中位數粒徑為3 mm與6 mm,幾何標準差為1.3~1.4。各測試微粒之質量遞減係數μ(ln(I/I0) / Mass,cm2/mg)以6 mm食鹽為-0.404最大,3 mm硝酸鉛為-0.968最小。而初步結果亦發現,在同微粒成分、同表面質量沈積密度情況下,細微粒所造成貝他射線遞減皆較粗微粒顯著,其相差可至10 %,初步原因推測貝他電子入射微粒時,細微粒貢獻較多表面積與較高電子屏蔽效應所致。Beta gauge與TEOM量測比較,一般以Beta gauge讀值較高,很可能是TEOM量測腔加熱,導致部分揮發性物質逸出。此外,在不加熱的狀態之下,Beta gauge在相對濕度超過75%時,即因為測試微粒NaCl的吸濕,而導致質量衰減係數的增加,此部分效應將利用類似SES的除濕系統持續探討。
沈積均勻度研究發現,當在固定沈積質量情況下,微粒沈積面積比例越小、或沈積層數越高,整穿透體射線強度上升。射源強度均勻度發現於射源邊際區域強度明顯下降。微粒沈積均勻時,其影響可忽略,但在沈積不均勻情形,則應列入考慮。
The beta gauge attenuation method of mass determination depends upon the near exponential decrease in the number of beta particles transmitted through a thin sample as the unit area density is increased. The performance of beta attenuation monitor can be suffered from several potential measurement artifacts, such as (1) particle size effects, (2) substrate inhomogeneity, (3) non-uniformity of filter deposit, (4) atomic number dependence, and (5) water vapor absorption by the aerosol deposits. The purpose of this work is to identify the principal factors resulting in overall inaccuracy, and hopefully to provide measure to remedy these artifacts.
A primitive beta gauge was built and tested in a calm air chamber. A constant output aerosol generator and an ultrasonic atomizing nozzle were used to produce challenge aerosols of different size distributions. An Aerodynamic Particle Sizer and a Scanning Mobility Particle Sizer combined system were used to measure the aerosol number concentration and size distribution, ranging from 5 nm to 30 mm. Beta ray passed through aerosol deposition on filter was recorded by a radiationmeter. TEOM and filter collection method were used to monitor aerosol mass concentration. The principal operational parameters include challenge aerosol size distribution, environment relative humility, filter location, amount of aerosol deposit, and uniformity of aerosol deposition.
The results showed that the beta intensity through the filter sample decreased exponentially with increasing aerosol deposit, as expected. The upper limit of surface mass deposition of the homemade beta attenuation system was estimated to be 0.8 mg/cm2. Among the challenge aerosols, lead nitrate has the highest attenuation (0.968 cm2/mg), followed by NaCl and then PST. Aerosols of smaller size appeared to attenuate more beta ray than the larger particles, a phenomenon needs to be studied further.
The comparison study between beta gauge and TEOM showed that beta gauge normally reads higher aerosol concentration than does TEOM, probably due to the fact that TEOM measures the mass under a heated environment; and therefore some of the material might vaporized and not to be measured by TEOM. Without the heating system, as if TEOM, beta gauge is likely to suffer the hygroscopic effect. To overcome this problem, a Sample Equilibration System (SES) developed for TEOM, should be effective in the beta attenuation system. The effect of uniformity of aerosol deposit on the beta attenuation methods could be significant. The Beta penetration increased with decreasing coverage (by aerosol deposit) given that mass of aerosol deposit was fixed.
摘要………………………………………………………………… I
Abstract…………………………………………………………….. II
目錄……………………………………………………………… IV
表目錄……………………………………………………………… V
圖目錄………………………………………………………… VI
第一章 前言………………………………………………. 1
第二章 文獻探討………………………………………………. 3
第一節 Beta gauge 之質量遞減機制……………………… 3
第二節 射源、濾紙與微粒特性對Beta gauge之影響……. 5
第三節 環境因素與系統設計對Beta gauge之影響……… 7
第三章 研究方法……………………………………………… 9
第一節 自製Beta gauge與實驗系統架設……………….. 9
第二節 射源穩定度與射源偵測器相對位置評估…….… 10
第三節 濾紙材質、濾紙厚度影響………………………. 10
第四節 粒徑分徑器效率評估……………………………. 10
第五節 微粒成分與微粒粒徑影響評估…………………. 11
第六節 濕度影響評估……………………………………. 11
第七節 自動關連程式研發………………………………. 11
第八節 戶外採樣比較……………………………………. 11
第九節 均勻度與沈積面積比例影響評估………………. 12
第四章 結果與討論……………………………………………… 13
第一節 射源穩定度與射源偵測器相對位置評估……….. 13
第二節 射源、濾紙材質、濾紙厚度、濾紙定位評估……… 13
第三節 粒徑分徑器效率評估與濾紙定位影響…………. 14
第四節 微粒成分與微粒粒徑影響評估………………… 14
第五節 濕度影響評估…………………………………... 15
第六節 Beta gauge量測值推導時間探討………………… 16
第七節 沈積均勻度與射源均勻度探討………………… 16
第八節 實地採樣比較…………………………………… 17
第五章 結論與後續工作……………………………………….… 18
第一節 結論……………………………………………….. 18
第二節 後續工作………………………………………….. 18
參考文獻………………………………………………………… 20
附錄………………………………………………………………… 22
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