Technical Report : DLJ/ TC/IL/97/7

 (A copy of the Technical Report released in 1997. )

ATOMIC EMISSION OF LIGHT

FROM SOURCES OF IONIZING RADIATION BY A

NEW ATOMIC PHENOMENON

Dr.M.A.PADMANABHA RAO

Defence Laboratory,

Jodhpur 342011, India

DEFENCE RESEARCH AND DEVELOPMENT ORGANISATION

MINISTRY OF DEFENCE

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ATOMIC EMISSION OF LIGHT FROM SOURCES OF

IONIZING RADIATION BY A NEW PHENOMENON

M.A. PADMANABHA RAO

Defence Laboratory, Jodhpur 342011, India

Abstract: Light emission has been observed from all the targets (like Rb, and Ba which are opaque to light during excitation by γ-rays from ²⁴¹Am), and the radionuclides investigated that can not be explained by the known phenomena. Light emission from metals ( Cu, Mo, and Ag targets; and radionuclides: ⁵⁷Co and ⁶⁰Co) at room temperature is a noteworthy observation, that implies the emission from metal atoms. It reveals that sources of ionizing radiation, in general, give rise to atomic emission of light by a new phenomenon. A detailed study showed dominance of ultraviolet(UV) radiation from low energy ionizing radiation sources, while raise towards near-infrared (NIR) radiation from high energy sources. These findings led to believe that ionizing radiation loose energy in eV level while passing through charged space within the atom of their origin. The loss of their energy that forms electromagnetic radiation, with energies in eV level, excites valence electron and causes fluorescent light emission.

1.INTRODUCTION

External incidence of ionizing radiation on certain materials causing scintillations, radioluminescence and Cherenkov light is well documented (Knoll, 1979; Goldberg and Weiner, 1989; Blasse and Grabmaier, 1994). The ionizing radiations are never believed to cause fluorescent light emission, within the atom of their origin, as it can not be explained by the known phenomena like Auger process or internal conversion (Compton and Allison, 1960; Knoll, 1979) for their energies being too high in keV or MeV level to excite a valence electron with binding energy in eV level. However, this paper reports on light emission observed from targets (excited by γ-rays from ²⁴¹Am), and radionuclides. Its poor intensity was the main reason why it missed from detection for a century since the discoveries of radioactivity and X-rays.

In fact, while studying phosphorescence of the double sulphate of potassium and uranium, radioactivity has been discovered by Henry Becquerel in 1896 (Becquerel, 1896). His observation on phosphorescence from the chemical compound differs entirely from the fluorescent light emission currently reported from radioactive atoms, like the uranium atoms. The light emission observed from radionuclides available mostly as radiochemicals such as ¹³⁷CsCl, differs also from very fast cross luminescence reported to have observed from CsCl on exposure to ionizing radiation at 300 K by Van Eijik (1993). In ¹³⁷CsCl, the ionizing radiations is claimed to cause light emission within the ¹³⁷Cs atoms, whereas in the later case of CsCl both Cs and Cl atoms participate in the scintillation mechanisms.

Importantly, light emission has been observed from metals ( Cu, Mo, and Ag targets during γ-excitation from ²⁴¹Am; and radionuclides: ⁵⁷Co, and ⁶⁰Co) at room temperature, while analysis of atomic fluorescence from metal atoms is known only when subjected to high temperatures (Nichols and Howes, 1924). It implies that the light emission takes place from metal atoms. This observation led to believe that, in general, the ionizing radiation sources give rise to atomic emission of light.

From each ionizing radiation source, light (UV, VIS, and NIR radiations) emission has been detected with the aid of a thin black polyethylene sheet, while the visible light (VIS), and NIR radiations with a pair of sheet polarizers (SP) that transmit visible and near-infrared range (Robertson , 1961). The results disclose that nature of its optical spectrum of an ionizing radiation source depends upon both the type and energy of ionizing radiations. Light emission observed from sources of ionizing radiation independent of the nature of source media, compelled to believe that ionizing radiation caused the light emission by an atomic phenomenon common to X-, and γ-ray, β-, and α-particle explained in this paper.

2. EXPERIMENTAL DETAILS

2.1 Sources and equipment

A variable energy X-ray source ( AMC 2084) and the reference sources ²⁴¹Am, and ¹⁵²Eu have been procured from Amersham International Plc, U.K. In the former, a sealed ceramic ²⁴¹Am source (γ-rays: 0.05954 MeV, 35.7 %) situated within a compact assembly excites a collimated (0.5 steradians) beam of characteristic X-rays from Cu, Rb, Mo, Ag, Ba, or Tb target mounted on a rotary holder (Fig. 2). Most radionuclides, with activities around 37 kBq, have been procured from the Board of Radiation and Isotope Technology (B.R.I.T), Bombay. An opaque mylar film fixed in front of these sealed reference sources was removed as it stops light from detection.

A Photomultiplier tube (PMT) of the type 9635QB (THORN EMI) with high gain (25 x 10⁶) known for detection of light and β-particles (Bohra et al., 1992) is demonstrated in the present study as a sensor even for X-rays, and γ-rays. Each source under study was kept directly over the quartz window of the PMT to enable detection of poor light emission from ionizing radiation sources. Simultaneous detection of light and ionizing radiations from these sources could be accomplished by setting gain of the linear amplifier high, and time constant at 0.1 μ sec. The PMT kept in a light tight metal encasing, provided with a detachable lid for facilitating frequent replacement of source etc., and further in a lead shielding was connected through associated electronics to a 8K MCA (Canberra). Since both ionizing radiation and light exhibited pulse height spectra devoid of any peaks in MCA, integral counts have been noted for four minutes, but shown in terms of counts per sec (cps) in Table 1, after making correction for background ( 13 cps) of the PMT.

The use of a 0.26 mm (28.2 mg cm^-2) thin polyethylene sheet suggested light emission from ionizing radiation sources, but direct proof of which has been provided by a pair of sheet polarizers (SP), which polarize light in the visible region, 400-710 nm. A verification of their transmission to UV radiation, on placing them in a beam of UV radiation, showed opacity upto 400 nm (Fig.1). On placing them in a beam of light, having set their polarizing axes parallel (parallel pair), they transmitted visible light (VIS) and near-infrared (NIR) radiation. On rotation of one of the sheets to 90, the pair failed to transmit visible light from 400 to 710 nm, while they transmitted as such the NIR radiation (unpolarized) beyond 710 nm. It is because that the parallel pair of SP transmit polarized light in the visible region 400-710 nm. But, when they were set as crossed pair, the first sheet transmits polarized visible light but the second one excludes from further transmission. This method was used in the present study to demonstrate light emission from ionizing radiation sources, and in the estimations of UV, VIS, and NIR radiations.

Fig.1. Transmission spectrum recorded of the pair of the sheet polarizers (SP) used in the study by a Double Beam Spectrometer (UV-150-02 from Shimadzu).

2.2 Interference of light produced in the PMT

For proper understanding of Figs. 2, and 3, it is essential to keep in mind that the characteristic X-rays emitted from the six targets such as Rb (Fig.2) and several radionuclides such as ¹³⁷Cs (Ba X-rays in Fig. 4); and the weak γ-rays from ²⁴¹Am, and ⁵⁷Co with energies below 0.330 MeV, the threshold limit for Cherenkov effect (Knoll, 1979), produce light in the quartz window of the PMT by molecular excitation. Whereas the γ-rays emitted with 0.330 MeV energy or above from radionuclides like ¹³⁷Cs (Fig.4), and the β-particles emitted with energies above the threshold limit of 0.156 MeV for Cherenkov effect (Bohra et al., 1992) from β-sources like ¹⁴⁷Pm to ⁹⁰Sr + ⁹⁰Y (as listed in Table 1) produce Cherenkov light in the window.

2.3 Method for measurement of light from Rb target:

Fig.2. A schematic diagram of the experimental set up used for measurement of light (UV, VIS, and NIR radiations) observed along with X-rays from Rb target during excitation by γ-rays from ²⁴¹Am.

Step (a): When the X-ray source was kept directly over the window of the PMT, in such a way that Rb target faces the window, 125,381 ± 22.9 cps have been observed (Fig.2, Table 1). It's pulse spectrum recorded for 4 min can be seen in Fig.3. Surprisingly these counts, being 28.5 times high over the Rb X-ray yield of 4400 photons/sec estimated by the manufacturer, could not be solely accounted to the X-rays that produce light in the window by molecular excitation.

Step (b): With a suspicion that the excessive counts could arise possibly by detection of light from the target, a 0.26 mm thin black polyethylene sheet was kept closely between the source and the PMT to see whether any counts fall because of exclusion of light, if any, by the sheet. As suspected, very low counts, 60 ± 0.5 cps noted have thus been caused by the detection of X-rays alone that passed through the sheet (Figs. 2, and 3). It is believed that the rest of the counts, 125,321 ± 23.4 cps (steps a-b) have been caused by light emission from the target (Table 1). The percent of light in the net counts caused by light and X-rays amounted to 99.95 %. But this could not be a confirmatory test for the light emission claimed.

Step (c): When the black sheet was replaced by a pair of sheet polarizers (SP), set as parallel pair, 535 ± 1.5 cps have been noted. These counts, higher than 60 ± 0.5 cps caused by X-rays in step (b), could be due to detection of VIS + NIR radiations transmitted by the SP as seen in Fig.1.

Step (d): On rotation of one of the polarizers to 90 (crossed pair) the counts fell to 72 ± 0.6 cps, due to exclusion of visible light from further transmission to the PMT, as discussed earlier on SP. It confirms light emission from the Rb target. Since crossed pair transmits NIR radiation (Fig.1), it is understood that these counts have been caused by the NIR radiation + X-rays transmitted by SP to the PMT, while the 535 ± 1.5 cps noted in step (c) have been caused by VIS + NIR radiations + X-rays. Hence the difference in counts, 463 ± 3.04 cps (Table 1) have been caused by visible light (VIS).

Step (e): When the black sheet was kept closely between the SP and the window to exclude even the NIR radiation, only 59 ± 0.5 cps have been observed, caused by the X-rays. Therefore, 13 ± 1.0 cps (steps d-e) have been caused by NIR radiation (Table 1). The reason for fall from very high counts in step (a) to step (c) has been attributed to exclusion of UV radiation by SP in step (c). From these findings, 125,321 ± 23.4 cps mentioned in step (b) to have been caused by light are thus due to UV+VIS+NIR radiations from the Rb target. Since the counts mentioned here include background level of 13 cps (Fig.3), the Rb X-rays in step (b) actually caused 47 ± 0.72 cps.

Fig.3. left Pulse height spectra recorded from 8K MCA. top: Background level 3200 counts / 4 min (13.3 cps) of the PMT served as a sensor to light, X-rays, γ-rays, and β-particles. middle: Light, and X-ray emissions from Rb target caused the counts of 3,00,91,447 /4 min (1,25,381 cps) as shown in step (a) of Fig.2. The X-rays could cause integral counts of only 14,343 / 4 min (59.7 cps) as shown in step (b) of Fig.2. The latter two readings included background level, 13.3 cps.

2.3 Method for measurement of light from ¹³⁷Cs

¹³⁷Cs emits not only Ba X-rays, but also hard γ-rays, and β-particles. Step (a): When the source was kept directly over the window of the PMT , 9098 ± 6.2 cps have been observed (Fig.4). From the experience of Fig.2, it is believed that these counts have been caused by UV+VIS+NIR radiations from the source, the Cherenkov light produced in the window by γ-rays of 0.662 MeV (85.0 %), and β-particles with E_max of 0.514 MeV (94.0 %) and 1.176 MeV (6.0%); and the light produced by molecular excitation in the window by Ba X-rays (K_α1 = 0.03219 MeV, 3.92%). Step (b): A 0.26 mm thin black polyethylene sheet, when kept closely between the source and the PM, excluded UV+VIS+NIR radiations but allowed the ionizing radiation to pass through and cause 2728 ± 3.4 cps.

step a: UV+VIS+NIR radiations+ionizing radiation = 9098 ± 6.2 cps

step b: ionizing radiation = 2728 ± 3.4 cps

steps (a-b): UV+ VIS+NIR radiation = 6370 ± 7.0 cps (Table 1)

step c: VIS+NIR radiations +ionizing radiation = 793 ± 3.6 cps (Fig.4)

step d: NIR radiation +ionizing radiation = 720 ± 3.5 cps

steps (c-d): visible light (VIS) = 73 ± 2.9 cps (Table 1)

step e: ionizing radiation = 519 ± 2.9 cps

steps (d-e): NIR radiation = 201 ± 2.3 cps (Table 1)

 3. RESULTS AND DISCUSSION

3.1 Light emission from X-ray sources:

  Table 1 evidently shows light ( UV, VIS, and NIR radiations) emission from Cu, Rb, Mo, Ag, Ba, and Tb targets (along with the characteristic X-rays excited by γ-rays from ²⁴¹Am), as well as from ⁵⁵Fe along with the Mn X-ray emission caused by electron capture. It reveals that their light emission, found nonspecific either to γ-excitation or electron capture, has been caused by X-rays.

 3.2 Light emission from metal sources:

An interesting observation has been the light (UV, VIS, and NIR radiations ) emission from metals present as Cu, Mo, or Ag targets or radionuclides like ⁵⁷Co (Mossbauer source), and ⁶⁰Co,

 Table 1. Visible (VIS), and near-infrared radiation (NIR), and their percentages in the light (UV+VIS+NIR radiations) measured from targets (during excitation by γ-rays from ²⁴¹Am ) and radionuclides in terms of counts per sec (cps).

The counts given here are after correction for background level of 13 cps. The sources are arranged in the order of increasing energy. Cu, Mo, and Ag targets, ⁵⁷Co* (a decayed Mossbauer source) , and ⁶⁰Co* are in metal form. Np L X, γ, and œ-emissions from ²⁴¹Am indicates Neptunium L X-ray, gamma and alpha emissions.notably at room temperature (Table 1), in contrast to the light emission known from a metal filament in an incandescent lamp.

In the former cases the energy required to do so is derived indirectly from ionizing radiations, while in the later by passage of electric current. Further, light emission has been observed even from opaque targets of Rb (125,321 ± 23.4 cps), Ba (2,064 ± 7.3 cps), and Tb (37 ± 1.1 cps) present in the form of salt (Table 1). In comparison, the poor emission from the metal targets can be evident from low counts that ranged from 22 ± 0.8 to 30 ± 1.0 cps, partly be due to the detection of light limited from their surface. The light emission observed from these eight opaque sources discloses that source medium has no play in it, hence no scope exists to attribute the emission to scintillations or Cherenkov light that requires a transparent medium (Knoll, 1979). On the other hand, since a pure metal source like Cu target comprises of only Cu atoms, atomic emission of light from the target can not be ruled out, in any case. Since the light has also been observed from Rb, Ba, and Tb targets, despite their opacity to light as in the case of metal targets, their atomic emission of light can not be ruled out, while such an emission from solids is known only when they were subjected to high temperatures (White, 1934). These strange observations compelled to believe that a new phenomenon is causing the atomic emission of light from ionizing radiation sources. In Table 1, ¹³³Ba, ¹⁵²Eu, and ²⁰¹Tl are regarded as predominant X-ray sources by virtue of their maximum X-ray abundance, hence their light emission is believed to have been caused mainly by X-rays.

 3.3 Light emission from beta emitters

Light (UV, VIS, and NIR radiations) has been detected even from ¹⁴⁷Pm, and ⁴⁵Ca, which are dominant beta emitters with 100% abundance in β-emission, and negligibly low (10^-3 to 10^-4 %) abundance in γ-, and X-ray emissions (Table 1). It appears that within a ¹⁴⁷Pm atom, β-particle causes the fluorescent light emission, as X-ray does in a ⁵⁵Fe atom. Since pure γ-, and œ-sources are not available, sought indirect evidences in the following to show that γ-rays, and œ-particles can also cause light emission.

 3.4 The new atomic phenomenon

The findings of the study led to an understanding on the new phenomenon. It is believed that ionizing radiation, while passing through charged space within the atom of its origin, looses energy in eV level that appears as electromagnetic radiation. The energy of the photons resulted thus exceeds the binding energy of valence electron so as to be sufficient enough for its excitation and causing fluorescent light emission. For example, ß-particles from ¹³⁷Cs ; the γ-rays, and Ba X-rays from its daughter ^137m Ba loose energy while passing through the charged space of the atom. Their loss of energy appear as electromagnetic radiation with different energies, each of which compete in excitation of valence electron. However, because of its maximum β-abundance (93.5%), its fluorescent light emission seems to have caused mostly by ß-particles (E_max=0.514 MeV) through this phenomenon, hence regarded it as a ß-emitter in Table 1, Figs 4, and 5.

At low energies, the number will be large of the ionizing radiations that loose energy as well as of the resulting photon population of electromagnetic radiation that excite valence electron and cause light emission. Evidently, the percent of light produced approaches nearly 100% from Rb (99.95%), ¹⁴⁷Pm (99.54%), ⁴⁵Ca(99.78%), and ²⁴¹Am (98.94%) in Fig.5, on estimation of the percentages from the counts caused by light (UV + VIS + NIR radiations) in the net counts (caused by light and ionizing radiations) given in Table 1. From this, the probability for this phenomenon to occur seems to be high at low energies. Low energy ionizing radiations loose relatively more energy, though at eV level. Consequently, the exciting photons of the electromagnetic radiation that resulted possess relatively high energy. Accordingly these exciting energies are expected to influence the nature of optical spectra of low energy sources by showing dominance towards short wavelengths, i.e with high percent of UV radiation, and correspondingly low percentage of VIS and NIR radiations. Evidently, the estimations made from Table 1 show low percentage of visible light (VIS), and further low percentage of NIR radiations in the net UV+VIS+NIR radiation emissions from low energy sources like Rb target (0.37%, 0.011%), ¹⁴⁷Pm (0.55%, 0.05%), and ²⁴¹Am (1.91%, 0.066%) which implies very high percentages of UV radiation emissions (99.619%, 99.4%, and 98.024% respectively).

Fig.5. Percent of light emission observed versus the energy of X-ray, γ-ray or β-particle emitted from each ionizing radiation source. The percent of light is estimated from the counts caused by the light (UV+VIS+NIR radiations) in the net counts (caused by light together with ionizing radiations) from Table 1. While plotting the graph, considered are the K_α1 energies of Mn X-rays (0.00589 MeV) from ⁵⁵Fe, Rb X-rays (0.013394 MeV) from Rb target, Cs X-rays (0.03097 MeV) from ¹³³Ba, Ba X-rays (0.032191 MeV) from Ba target, Sm X-rays (0.040124 MeV) from ¹⁵²Eu, and Hg X-rays (0.07082 MeV ) from ²⁰¹Tl, because of maximum percentage of X-ray abundance in the case of these radionuclides. E_max is considered because of maximum β-abundance in ¹⁴⁷Pm (0.224 MeV), ⁴⁵Ca (0.252 MeV), ¹⁴¹Ce (0.444 MeV), ¹³⁷Cs (0.514 MeV), ¹³¹I (0.607 MeV), ²⁰⁴Tl (0.763 MeV), ⁸⁶Rb (1.77 MeV), and ⁹⁰Sr + ⁹⁰Y(2.27 MeV of ⁹⁰Y). The γ-energies are considered because of maximum γ-abundance in ²⁴¹Am (0.05954 MeV), ⁵⁷Co (0.122 MeV), ¹¹³Sn (^113mIn γ: 0.393 MeV), ²²Na (0.511 MeV); ^110mAg (0.658 MeV), ⁵⁹Fe (1.099 MeV), ⁶⁰Co (1.33 MeV). Please refer the text for reason to select γ-ray energy instead of α-energy with maximum abundance.

Fig.6 Ratio of NIR/ VIS radiation emissions versus the energy of X-rays, γ-rays, or β-particles emitted by each ionizing radiation source investigated. Refer Table 1 for contribution of VIS and NIR radiations, and the footnote under Fig.5 for the ionizing radiation energies.

At higher energies, large number of ionizing radiations escape through space within the atoms of their origin with no loss of energy. Only a few will loose minimum energy at eV level that results in forming a thin photon population of electromagnetic radiation, with relatively low energy in eV level, available to excite valence electron. For this reason, a fall in percent light with increase in energy of ionizing radiation in the case of X-, γ-, and β-sources can be evident in Fig.5. Moreover, fall in percent of light from X-ray sources seems to have a trend different from that of γ-, and ß- sources suggesting that the percent light emission from a given source depends upon both the type and energy of its ionizing radiation emissions.

As the energies of ionizing radiations approach or cross 1 MeV level, as said earlier, the resulting thin exciting photon population shall have relatively low energy, in eV level. Consequently, the resulting spectrum is expected to have a rise in contribution towards longer wavelengths (NIR radiations) with a corresponding fall in UV, and VIS radiations. Evidently, raise in the ratio of NIR/VIS radiations (Table 1) can be seen in Fig. 6 as proceeded from low to high energy sources, with apparently a trend for X-ray sources such as Rb target to ²⁰¹Tl; different from that of ¹⁴⁷Pm to ⁹⁰Sr+⁹⁰Y; or ²⁴¹Am to ⁶⁰Co as listed in Table 1. These results confirm the findings of Fig.5 that the nature of optical spectrum of a ionizing radiation source depends on the type and energy of ionizing radiation. Reasonably it could happen only by a new phenomenon.

It is seen from Fig.5 that high energy ionizing radiations caused low percent light. From ⁶⁰Co, available as radiochemical (⁶⁰CoCl), the low percent light (74.3%) estimated thus suggests to have been caused by high energy ionizing radiation particularly the γ-rays of 1.1732 MeV (100%), and 1.3324 MeV (100%) energies. In exciting the valence electron, the γ-rays with 200% abundance compete with low energy β-particles (E_max = 0.315 MeV) having only 99.74% abundance, hence probability for excitation of valence electron by γ-rays appears high (Ref: Lederer and Shirley, 1978)). Light emission from ⁶⁰Co exemplifies that γ-rays also cause light emission.

Since œ-particles from ²⁴¹Am have maximum abundance (85.2%) the percent light should infact reflect their 5.486 MeV energy , as has been the case with all other sources. As their energy is nearly twice to that of the β-particles (E_max=2.27 MeV) from ⁹⁰Y, the percent light from ²⁴¹Am is expected to be far below that of ⁹⁰Y (53.26%). But, at that energy it is likely that most α-particles escape from ²⁴¹Am with no loss of energy, and a few may loose very minimal energy in eV level. It is possible that only a few photons of the electromagnetic radiation resulted shall be available for excitation of valence electron, hence probability for their interaction can be very low. In comparison, the probability for excitation of valence electron appears high for γ-rays (0.05954 MeV) because of their maximum abundance (35.7%) over Np L-series X-rays (L_α=13.93 keV , 24.4%, Radionuclide transformations, 1983). High percent light (98.94%), low percent VIS (1.91%), and NIR (0.066%) radiations estimated from Table 1 supports the view that the light emission has been caused mainly by γ-rays, while contribution of œ-particles in the light emission can be relatively low.

In the present study light has been observed from sources like ¹³¹I, and ¹³⁷Cs with negligibly small media that can not be seen, and from opaque targets present in the form of salts, or metals revealing that the emission is independent of source medium, a fact which was not known earlier. It could be possible only when light photons follow X-rays, γ-rays, β-particles, and œ-particles from the atoms of their origin. This study reveals that excited atoms present in a X-ray target emit characteristic X-ray photons in keV level followed by fluorescent light photons (Fig.2) with energy in eV level before their returning to ground state, departing from the conventional belief. This has a relevance in atomic physics. Similarly, emission of light photons following that of β-particles from unstable atoms of a radionuclide, such as ¹³⁷Cs atoms and further emission of light photons following that of γ-rays and Ba X-rays from its daughter ^137mBa atoms (Fig.4) prior to their return to stable state as ¹³⁷Ba atoms has significance in nuclear physics.

 4.CONCLUSIONS

  The novel findings of the current study have relevance to nuclear and atomic physics. Light emission has been observed from ionizing radiation sources such as targets (during γ-excitation), and radionuclides. Therefore it is realized that an atom in a X-ray target or a radionuclide not only emit ionizing radiation but also fluorescent light prior to its return to the designated ground or stable state. The light emission observed from metal sources such as targets of Cu, and Mo, and ⁵⁷Co and ⁶⁰Co notably at room temperature reveals atomic emission of light from ionizing radiation sources, in general. At low energies, ionizing radiations caused an optical spectrum dominated by UV radiation with a very low contributions of VIS and NIR radiations. At high energies, a rise in contributions towards NIR radiation, and a corresponding fall in contributions of UV and NIR radiations has been evident. The study shows that the type and energy of ionizing radiation effect the nature of optical spectrum. From these findings, a new phenomenon causing the light emission from ionizing radiation sources is explained here.

 Acknowledgements: The author gratefully acknowledges the kind assistance rendered by colleagues, Mr. Dinesh Bohra, and Mr. Arvind Parihar in conducting some experiments initially.

  5. REFERENCES

1. Becquerel H. (1896), Sur les radiations invisibles emises par les corps phosphorescents, Comptes rendus de l' Academie des sciences, Paris, 122, 501-503. (translation) In G.T.Seaborg and W. Loveland (Eds) Nuclear Chemistry, Benchmark Papers in Physical Chemistry and Chemical Physics V. 5 (1982) Hutchinson Ross Publishing Company, Pennsylvania, pp. 23-25.
2. Blasse G. and Grabmaier B.C (1994), Luminescent materials, Spring-Verlag, New York, pp. 170-194.
3. Bohra D. Parihar A. and Padmanabha Rao M.A. (1992) The photomultiplier as a beta detector, Nucl. Instr. and Meth. in Phy. Res. A 320, 393-395.
4. Compton A.H. and Allison S.K. (1960) X-rays in theory and experiment, D. Van Nostrand Co, Inc, New York, pp. 211-220.
5. Goldberg M.C. and Weiner E.R. (1989) The science of luminescence, In M. C.
6. Goldberg (Ed) Luminescence Applications in Biological, Chemical, Environmental, and Hydrological Sciences, American Chemical Society, Washington, DC, pp. 1-22.
7. Knoll, G.F. (1979) Radiation Detection and Measurement , John Wiley & Sons, New York, pp.745-748.
8. Lederer C.M. and Shirley V.S. (1978) Table of isotopes, Seventh Edition, John Wiley Sons, Inc., New York.
9. Nichols E.L. and Howes H.L. (1924) Phys. Rev. 23, 472.
10. Radionuclide Transformations, Energy and Intensity of Emissions (1983) ICRP Publication 38, Pergamon Press, Oxford.
11. Robertson J. K.(1961) Introduction to optics, Geometrical and Physical, D.Van Nostrand Company inc, Toronto, pp 259-270.
12. Van Eijk CWE.(1993) Fast scintillators and their applications. Nuclear Tracks Radiat. Meas. 21, 5-10.
13. White H.E. (1934) Introduction to Atomic Spectra, Mc Graw-Hill Book Co, Inc, New York.

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