طيف‌سنجي لامپ‌هاي سرخ و استفاده از نورتابي در بررسي مناسب بودن منبع نوري آزمايشگاه آماده‌‌سازي نمونه‌‌هاي سن‌‌يابي به روش OSL در ايران

Investigation of suitability of installed red light in the OSL sample preparation laboratory in Iran using spectrometry and luminescence methods

ايران از معدود كشورهايي است كه علاوه بر قرار گرفتن بر كمربند زلزله جهاني، داراي قدمت ديرينه است. در طول تاريخ زلزله مهم‌ترين خطر طبيعي براي ايران و ساكنان آن بوده است. بسياري از تمدن‌ها، شهرها، روستاها و آثار باستاني ايران زمين در اثر حوادث طبيعي مثل زلزله سيل و آتشفشان نابود شده يا صدمه ديده‌اند. لذا امكان تركيب اطلاعات ديرين‌‌زلزله‌‌شناسي، باستان‌‌شناسي و ديرين‌‌اقليم‌‌شناسي در كشورمان فراهم است. اين تركيب نه تنها اطلاعات جامع علمي در هر سه زمينه پيش‌گفته را فراهم مي‌آورد بلكه فهرست‌نامه سني حوادث طبيعي ايران را كامل مي‌كند. كليد تركيب ديرين‌‌زلزله‌‌شناسي، باستان‌‌شناسي و ديرين‌‌اقليم‌‌شناسي، سن‌‌يابي آثار حوادث گذشته است كه در دل خاك مدفون شده‌اند. نورتابي (لومينسانس) روشي مناسب براي سن‌‌يابي در محيط‌هاي خشك و نيمه‌‌خشك است. نورتابي مستقيما آخرين نورخوردگي مواد معدني همچون كوارتز و فلدسپار را كه به وفور در خاك يافت مي‌شوند، تعيين سن مي‌كند. با توجه به اينكه به‌منظور سن‌‌يابي به روش نورتابي از نور مريي استفاده مي‌شود و مي‌بايد نمونه از زمان خروج از خاك تا تحريك آن با نور كنترل شده مريي در آزمايشگاه، تحت تاثير هيچ نور مضري قرار نگيرد، موضوع نصب نور امن در آزمايشگاه امري حياتي به‌شمار مي‌رود. اين مقاله ضمن توضيح نقش سن‌‌يابي و دُزسنجي به‌روش نورتابي در ايران به تشريح چگونگي بررسي امنيت نوري، شامل تهيه و نصب چراغ سرخ و آزمون طيف نور با طيف سنج و اثر نور بر سيگنال طبيعي نورتابي مي‌پردازد.

Iran is one of the limited numbers of ancient countries which is almost totally located on earthquake belt. Earthquakes have always been the most important natural hazard in Iran. Many villages, cities, civilizations and monuments have been partly or totally destroyed by past earthquakes in Iran. As a developing country, Iran is expanding its cities, industries, dams, power plants etc. The recent earthquake in Japan showed the effect of an earthquake and tsunami on the nuclear power plants. After the failure of Park field earthquake prediction (based on probabilistic approach), we should think carefully about the importance of deterministic approaches for earthquake hazard assessment in Iran. Therefore, Iran requires mapping its earthquake faults and investigating the activity of faults as first step. We should define the zones and find the codes based on the real data and parameters. Slip rate is one of the most important parameters for natural hazard risk assessment. For slip rate determination we should measure the slip of the fault and the time that has taken for this slip to occur. Therefore, we require a suitable dating method. Meanwhile, Iran is arid, semi arid zone. These make luminescence as the most suitable method for dating past earthquakes, civilizations and environment. Luminescence dating can join studies related to the paleoclimatology, Paleoseismology and archeology of Iran. Optically stimulated luminescence (OSL) has been employed so far for dating the time of recent earthquakes and determination of the slip rate of some important earthquake faults in Iran (e.g., Fattahi et al., 2006; 2007; 2009; Fattahi and Walker, 2007). OSL directly dates the last time that quartz and feldspar in a sample has been exposed to light. Therefore, if we can find a sample that has been exposed to light during an event, we can date that event. The event can be an earthquake, Tsunami, volcanic eruption, sudden environment change and etc. Therefore, OSL samples should not be exposed to light during sampling and sample preparation. For this reason the suitability of the light of the sample preparation lab is vital. It is not possible to work in complete darkness, but dim red light with specific wave length can be employed for quartz preparation. Therefore, in the OSL sample preparation room in the Institute of Geophysics, normal red light which is available in the market, was installed and tested. For this purpose the spectrometer of red lamps with and without available filters were tested. The suitability of the dim light in the lab was also tested using Equivalent dose (De) estimate of well behaved quartz samples. Quartz grains were divided into different parts. One part was kept in complete darkness, the second was exposed to the lab light on the work plates and a third part was put in the fume cupboard. The Equivalent dose (which is the lab dose equal to the natural dose) of these three parts was estimated. Unfortunately, the Equivalent dose of grains that had been exposed to the lab light underestimated the natural dose. The result of all above mentioned experiments showed that the installed light was not suitable.