Study of gamma spectrometry laboratory measurement in various sediment and vulcanic rocks

Gamma-ray spectroscopy is the quantitative study of the energy spectra of gamma-ray sources. This method is powerful to characterize some minerals, especially to differentiate rocks which contains among Potassium, Uranium, dan Thorium. Rock contains radioactive material which produce gamma rays in various energies and intensities. When these emissions are detected and analyzed with a spectroscopy system, a gamma-ray energy spectrum can be used as indicator for mineral content of rock. Some sediment and vulcanic rock have been collected from East Java Basin. Samples are ranging from Andesite vulcanics, Tuff, Shale, various vulcanic clay and Alluvial clay. We present some unique characteristics of gamma spectrometry in various sedimentar and vulcanic rocks of East Java Basins. Details contents of gamma ray spectra give enrichments to characterize sample of sediment and vulcanic in East Java. Weathered vulcanic clay has lower counting rate of gamma ray than alluvial deltaic clay counting rate. Therefore, gamma spectrometrometry can be used as tool for characterizing the enviroment of clay whether vulcanic or alluvial-deltaic. This phenomena indicates that gamma ray spectrometry can be as tool for characterizing the clay whether it tends to Smectite or Illite


Introduction
The database information of gamma radiation of rocks is needed for many purposes such example as radiation measurement and dose rate of tiling rocks [1], identification of low-grade uranium (U) ore with thorium resources in a large syngenetic deposit using gamma spectrometry borehole logging [2], detection of potassic alteration to mineralization [3] and challenge discrimination of clay among Illite, Smectite and Kaolinite.
Some authors measured gamma spectrometry to characterize radioactivity of rock and soils [4,5,6]. In this work, sample rocks collected in East Java Basin are measured to initiate the databases of gamma spectrometry in various sediment rock in Indonesia.
Gamma spectrometer was used widely to identify radionuclide gamma emitter especially to discriminate among Thorium, Uranium and Potassium [7]. As well as Si or Li detectors [8], the scintillation detector NaI (T1) is detector that compose with material that could makes flash light if interact with gamma radiation. In detector, gamma ray makes photoelectric effect, Compton scattering, and pair production. Light intensity as output scintilator crystal proportion with gamma ray energy. Flash light is transmitted to photocathode from photomultiplier tube (PMT). Electric current from 1 PMT makes pulse in preamplifier. The pulse then is processed with Multichannel Analyzer Analyzer (MCA). Pulse amplitude is proportion with gamma energy.
There are three radionuclide series, Uranium, Thorium, and Potassium series. All kinds of rocks have uranium, especially granite where one ton granite has about three until four gram Uranium. Uranium generally has homogeny distribution in rock. Mainly uranite has 80% and euksinit has 20%. There are uranium isotop with abundances 99,275% (238U), 0,72% (235U), and 0,005% (234U) [8] There is small amount of Thorium in rocks and soil. In soil Thorium amount about 12 ppm with 232Th isotope has abundance 100%.
Radionuclide detection is easility detected by gamma spectrometer, the uniqueness of energy spectrum can discriminate among Uranium, Thorium, and Pottasium concentration [9] [10]. In this work, gamma ray spectrometer is used to detect the existence of Potassium, Uranium, and Thorium from sample of rocks in East Java Basin

Experimental Method
The samples of sediment rocks were collected from northern part to southern part of East Java Figure  1.

Results and Dicussion
Counting time is choosen 600 second. Figure 3 and 4 shows the counting curve vs energy (in keV) of ten sample of rocks. Type of sample covers: vulcanic claystone, vulcanic tuff, weathered claystone and alluvial clay. The vulcanic claystone, culcanic sand, weathered claystone were collected from Cangar, near Malang. Some samples of claystone where collected near Jolotundo in leg of Mt. Pananggungan. Some samples of vulcanic tuff and vulcanic "alluvial" were collected from Songgoriti and leg of Mt.Welirang, The detail of sample position is listed in Table 1. Table 1 shows tabulation of total countingrate. The sample of rocks is sorted by countingrate value from higher to lower. The result of data processing from ten sample also is showen by figure 3

Conclusions
The difference of gamma ray spectra among Sandstone, Claystone, Tuff, Alluvial, Weathered clay, Shale/clay is significant. The rock sample collected from East Java Basin has lower Uranium and Thorium's spectra compared than Postassium's spectra. Weathered vulcanic clay has lower counting rate of gamma ray than alluvial deltaic clay counting rate. Therefore, gamma spectrometrometry can be used as tool for characterizing the enviroment of clay whether vulcanic or alluvial-deltaic. This phenomena indicates that gamma ray spectrometry can be as tool for characterizing the clay whether it tends to Smectite or Illite.