Quality Control Checks In OSL Study Of Old Banyan Trees In North Region Of India

Since two decades, Optical dating is establishing a popular absolute method of dating using sedimentary grains like quartz and fledspar as dosimeters. The age estimation is based on the time elapsed since last exposure of sunlight with the grain under study. Sunlight either direct or indirect is capable of bleaching the sediments. Some characteristics of the quartz and fledspar like the defects in their crystals enables them to trap the charges at the defects sites which act as trapping centers .These traps get filled as an action of ionizing radiation coming from long lived radioactive nuclides present in the environment. More the dose from ionizing radiation, more will be the trapped charges(Aitken 1998)

Study Area and sampling

The monumental ‘Heritage Banyan tree’ tree under  study is having  length of about 55 feet with the trunk circumference of 27 feet. It has latitude of 30.7482N and longitude of 76.7658E and it is at 366m altitude. Another site of study is Banyan tree study is located in the village Dhani Majra of Fatehabad.The latitude of the place is 29.49⁰ N and the longitude is 75.53⁰E. The region is classified as tropical desert, arid and hot. It’s location is Indo-gangetic plains and  the  gradient of .27m/km of master slope of the land is towards south west.

For sampling of sediments the author dig a pit of about 1 meter. The roots of tree got exposed during digging and the homogeneous profile at a depth of 1 meter beneath the roots for first site and 0.5 m for second site was selected for inserting the sampling tube. The cylindrical opaque sampling tubes of stainless steel having a length of 30 cm and diameter of 5 cm with Styrofoam plug were used for sampling. The horizontal samples were collected obeying the sampling protocols for luminescence dating.

Age Estimation

The dose acquired by the dosimeter and annual dose rate measurement helps in estimating the age of the sedimentary grain. , these two terms are essential measurements for the study. Equivalent dose measurement is done by adopting the SAR protocol (Murray and Wintle 2006).For annual dose rate determination ,gamma spectroscopy using High Purity Germanium detector is used.

Quality Control checks

Luminescence produced by stimulation of grains is characterized by the composition of the related grain i.e. the defects present in the grain determines the amount of luminescence. The prerequisite of precise age measurement, protocol adopted must be able recover the actual dose received by the sample during burial. The sample should meet all tests in the laboratory for supporting the protocol.

 While working with optical dating, various parameters need to be checked to obtain precise age estimation. In the present study ,a number of quality control tests have been employed  to ensure suitability of  the adopted protocol.

Purity test

To work with one type of grain it is required that is must not be contaminated with other grains present in the sediments. Contamination may lead to imprecise dating. Since this is most likely issue so various methods have been adopted to check this contamination before using the sedimentary grain for Optical dating. Out of myriad of grain types present in sediments, quartz and fledspar are two most common and present in abundance. Consequently, while working with OSL, fledpar contamination is common. But the fledspar luminescence with OSL is more intense as compared to quartz luminescence(Aitken 1998). The exposure of sample containing both (quartz and fledspar)with Infrared light(850 nm),maximum contribution in obtained luminescence is from fledspar and quartz contribution is rare(Aitken 1998).Thus Infra red stimulated luminescence when compared with blue light  stimulated luminescence(for quartz stimulation),more IR luminescence indicates the contamination of quartz with fledspar. However, the sample having 10% ratio of Infra red to blue signal are acceptable (Alexanderson. et al 2008).

However,the fledspar contamination is not much problematic due to its brighter luminescence as compared to that of quartz but its evaluation is needed.Myraid of techniques are available for the removal of unwanted fledspar from the sample which are followed in the sample preparation procedure like treatment of sample with heavy liquid anf then with the HCl.These techniques only help in getting rid of macroinclusions but the microinclusions are needed to be tackled in some other way like double SAR protocol (Banerjee et al., 2001).For both the samples CHD-03 and FTB-01 the purity test has been followed and to select the aliquots, prepared samples are placed in the sample holder to follow the steps for purity test as demonstrated in table 1.The signals that are received during this test are shown in fig(a) and (b) for both the samples.

Table(1) steps for purity test.

Table(2)  Summary of values of ratio of IRSL/Blue for both the samples. Ratio depicts the feldspar concentration. Below 10% concentration values are accepted.

(a)

(b)

Fig(a) comparison of OSL counts of the sample for natural and laboratory dose having a sharp peak in decay curve representing intense luminescent OSL signal for sample CHD-03 fig(b) for sample FTB-01

Recuperation and recycling ratio test

The most accurate procedure for measuring Palaeodose is SAR protocol(Murray and Wintle 2006) whose performance is evaluated by two tests which are the part of the SAR sequence.

For recuperation test ,SAR cycle is performed by zero regenerative dose and L₀ is measured and then a test dose is given for which T₀ is observed.This ratio of L₀/ T₀ gives the recuperation. Theoretically, this ratio should able to put the growth curve at origin. More recuperation value indicates the presence of undesirable signal due to preheating of the sample i.e. a signal even in the absence of ionizing radiation. Experimentally, the presence of a small signal which is not exceeding the natural signal by 5% is insignificant (Murray and Wintle, 2000).It is desirable that R₄/N is less than 5%. But the recuperation signal if exceeds this prescribed value then either the aliquot is not suitable or the protocol is modified with different preheat temperature( Murray and Wintle, 2003).The recuperation obtained for the sample CHD-03 is 0.07 and for FTB-01 is 0.9 . Both the values confirms that the adopted protocol is significant.

The recycling ratio test is also an another parameter for ensuring the applicability of the protocol. This test will be significant only when the final regeneration dose is equal the first regeneration dose (Wintle and Murray, 2006). For the adopted protocol to be appropriate, L_i should be proportional to T_i for constant D_i. which means that the corrected sensitivity signal is not dependent on the prior thermal or dose treatment. (Wallinga et al., 2000).  To check this,  the same regenerative dose is repeated as given in the first cycle and in both the cases ratio of (R_i= L_i/  T_i)   ratio is compared. Since the dose on two cases is same so this ratio should be same .In the present study, this ratio is administered while the construction of the growth curve .When the dose is repeated for first and fifth cycle then the ratio of R₅/R₁ is found to be 0.94 for CHD-03 sample and 0.98 for FTB-01 sample which lies in the range 0.91-1.15.(0.9< R₅/R₁<1.1)(Murray and Olley, 2002)These values of recycling ratio which are in the range supports the choice of SAR protocol.

Dose recovery test

This is another deciding parameter for confirmation of correct SAR protocol adoption. For this test, the sample is optically zeroed followed by administrating a known laboratory dose using dose source in the laboratory. Then this laboratory induced dose is recovered by adopting the selected protocol starting with preheating. The protocol which recover this induced dose correctly is suitable for the measurements.

Fig: Results of the preheat temperature versus dose-recovery performed for quartz

Although the accurate dose recovery dose not assures the correct measurement of natural dose but this test is helpful in selecting the suitable protocol for the precise measurement. Theoretically, the ratio of dose measured to the dose induced should be one but in practice , the  ratio between range 0.90-1.10 is fairly good. For the samples studied the ratio is found to be 1.02 at the temperature 240 ºC which confirms the protocol suitability for the selected samples.

Thermal Transfer (TT) test

The occupation of light sensitive traps due to the movement of charges from light insensitive traps as a result of heating(preheating) (Wintle and Murray, 2006) causes the thermal transfer. Deeper traps electrons which are likely to be not properly bleached when they get shifted to light sensitive traps ,they participate in the OSL Luminescence emission. This causes the overestimation of age. As thermal transfer is temperature dependent . Generally young sediment samples are more prominent to thermal transfer so this assure test needs to be done before selecting the aliquots for luminescence. For performing this test the aliquots containing the grains are bleached in the laboratory. After bleaching  if any dose is still contained in the grains so this indicates the thermal transfer. However, in the current study, sample CHD_03 and FTB_01 when tested at preheat temperature 240 ⁰C for the different regenerative doses it is concluded that thermal transfer is not much significant at the selected preheat temperature.

CONCLUSION

The age estimation of both the samples is  in well agreement as given by the related agencies which supports the suitability of the adopted procedures and the quality control tests performed a are also in support of the OSL method of dating for the alive Banyan trees under study.

REFERENCES

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