Suren Kandasamy Dissertation.pdf - University of Surrey

Number of countsM.Sc Dissertation4.7 Internal RadioactivityNaturally occurring lanthanum (La) is composed of one stable ( 139 La) and oneradioactive ( 138 La) isotopes with half life of 1.06×10 11 years. 139 La is the most abundantisotope (99.91%) and 138 La is very rare isotope. 138La decays both by electron capture(EC) decay (66.7%) and beta decay (33.3%). The EC decay is to a 1436 keV excitedstate in 138 Ba, which de-excites through a single 1436 gamma transition. The lesspreventbeta branch has a Q value of 1044 keV and populates the 789 keV state in 138 Ce,which decays through a single gamma transition to the 138 Ce state [9].10000090000800007000060000Beta continuum1436 Gamma+32 keV X-ray5000040000789 Gamma+Beta continuum30000200001000000 200 400 600 800 1000 1200 1400 1600Figure 4.10 Self-counting background spectrum from the LaCl 3 :Ce detector.The values of peak indicate their original values Ref [5]. But these values areslightly different with our calibrated systems.One of the major disadvantages of the lanthanum halide scintillator is their internalradioactivity. The background (internal radioactivity) spectrum easily measured by selfcounting for long time period. Fig. 4.10 shows such a spectrum collected byLaCl 3 :Ce(10%) with the size ofEnergy (keV) 44.450.8mm, BrilLanCe B350 crystal, countedfor about 2 days (154110 sec) in a very well shielded chamber (led castle). The energyaxis of the Fig. 4.10 shows the calibrated systems energy scale. Because of that the peakenergy values are shifted by some values, but the shape of the spectrum is similar to thestandard background spectrum of LaCl 3 :Ce crystal. For this period the total number ofnet counts per second/cm 3 was calculated, which is equal to 1.17 cps/cm 3 .K. Suren 29