Optical and electrical properties of gamma irradiated PADC detector

PADC (Polyallyldiglycol carbonate) detector subjected to gamma rays undergoes change in optical and electrical properties. The optical band-gap is determined using Tauc's plot. The band-gap is found to be unchanged up to the dose of 10³ Gy, and then starts decreasing with increasing dose. The band-gap is lowered from 4.04 eV for the pristine material to 2.46 eV for the highest dose (10⁶ Gy). The dielectric constant of the detector is influenced due to the gamma exposure. At lower doses (up to 10⁴ Gy), the detector seems to be not polarised, so no change in dielectric constant is observed. But at the dose of 10⁵ Gy, the value is increased slightly while at the highest dose (10⁶ Gy), the dielectric constant of the detector is found to be enhanced significantly. The results indicated a decrease in track registration sensitivity with increase in gamma dose.     

Effect of high gamma dose on the PADC properties

The effect of the properties of PADC nuclear track detectors after exposure to high doses of gamma absorbed doses up to 5×10⁵ Gray (50 Mrad) were studied. The gamma source was a 9.03 PBq (244 KCi), Co-60 source. Results indicate that each of the bulk etch rate (V_b), the tracks etch rate (V_t) and the sensitivity (V) of the detectors increases with the high gamma absorbed dose, but there is a drop in these parameters at the low gamma absorbed dose. Signs of surface roughness were observed by increasing the gamma absorbed doses and changes in color observed for doses larger than 2×10⁵ Gray. The temperature of detectors during irradiation time reached 41°C. The fission fragment tracks (from Cf-252 source) disappeared quickly within the etching time (minutes) for total absorbed doses greater than 3×10⁵ Gray due to their high bulk etch rate.     

Effect of gamma rays on PADC detectors

The effect of Gamma radiation in Polyallydiglycol carbonate (PADC) detectorrs has been studied in the dose range of 10⁰–10⁶ Gy. Some of the properties like bulk-etch rate, track-etch rate, activation energy for bulk and track-etching have been found out for different gamma doses from ⁶⁰Co Source in PADC. The experimental results are presented and discussed.     

Gamma ray photon-induced modifications in Triafol-TN and Triafol-BN polymeric track detectors

Photon induced modifications in Triafol-TN and Triafol-BN polymers have been studied in the dose range of 10¹–10⁶ Gy at room temperature using a ⁶⁰Co source. To monitor the chemical and structural changes induced by gamma rays, UV, IR, and ESR studies were carried out. Thermal studies were also conducted for understanding the effects of gamma irradiations on these polymers. Variation of track etching characteristics and activation energy for bulk etching have been studied at different gamma rays doses. The experimental results are presented and discussed.     

Modifications in track registration response of PADC detector by energetic protons

It has been well established that different ionising radiations modify the track registration properties of dielectric solids. In an effort to study the response of Polyallyl diglycol carbonate (PADC Homalite) detector towards fission fragment, PADC detectors were exposed to 10⁴ Gy dose of 62 MeV protons and then one set of samples were exposed to fission fragments from a ²⁵²Cf source. Two of these detectors were containing a thin layer of Buckminsterfullerene (C₆₀). The study of the etched tracks by Leitz Optical Microscope reveals that the track diameters are enhanced by more than 70% in the proton irradiated zone as compared to that in the unirradiated zone. Scanning Electron Microscopy was performed after etching the sample in 6 N NaOH at 55°C for different etching times, to study the details of the surface modifications due to proton irradiation of PADC detectors with and without C₆₀ layer. Our observations revealed that the diameters and density of proton tracks have increased with etching time on the surface facing the fullerene layer as well as the other surface. However, a relatively more open structure of the etched surface containing C₆₀ as compared to the bare one may be an indication of the extra damage caused by the energy released upon the destruction of C₆₀ molecules by energetic protons.     

Environmental gamma and radon dosimetry in Venezuela

Environmental gamma exposure and radon concentration levels measured in Venezuelan regions are presented. A new generation image analyser was used for particle track counting in CR-39 detectors. Mineral water wells from where water is supplied for massive consumption have an alpha activity around 0.450 Bq L⁻¹ and few of them have concentrations above 50 Bq L⁻¹. Coastal potable water activity is on the average around 5.3 ± 12% Bq L⁻¹. Indoor radon national average is 36 ± 5% Bq m⁻³; in two of the 36 monitored sites, the measured average is above 400 ± 5% Bq m⁻³. In air gamma dose values are between 100 and 144 nGy h⁻¹. In soil, ¹³⁷Cs concentration is around 0.5 and 10 Bq kg⁻¹ at the depth of down to 20 cm. Building materials were included in this study. ⁷Be and ¹³⁷Cs were measured in low concentration in tropical plants on Tepuy-s (sacred mountains in the Amazonas State). Geological active faults were identified by radon concentration measurements using LR-115.     

TL and PTTL in natural fluorite previously irradiated with gamma rays and heavy ions

The effect of annealing temperature on the phototransfer thermoluminescence (PTTL) signal was studied to determine the appropriate annealing temperature for treating the natural powder before irradiation. The temperatures used to anneal virgin natural fluorite samples (only natural dose without giving the samples any artificial doses) were 150, 250, 350, 450, 550, 650 and 750°C for a duration of 1 h in each case. The results show that the PTTL response did not change for anneal temperatures up to 450°C, but at higher temperatures the signal decreased rapidly. The height of the 90°C peak decreased by two orders of magnitude as the anneal temperature increased from 450 to 750°C, whilst the height of the 180°C peak decreased by three orders of magnitude between the same two annealing temperatures. In order to investigate the effect of previous gamma rays and heavy ion irradiation on thermoluminescence (TL) and PTTL signals, powdered samples of natural fluorite from Cornwall, England, were annealed at 500°C and then irradiated (at GSI, Darmstadt, Germany) with ¹⁶¹Dy ions of energy 13 Mev/n; the range of fluences used was from 10⁴ to 10¹² ion cm⁻². Identical samples were given gamma doses in the range 1 Gy to 2.6 × 10⁴ Gy in order to compare the effects of gamma rays and heavy ions. The sensitivities of TL and PTTL were studied by giving the samples a gamma test dose of 1 Gy after annealing the samples at 500°C for 30 min in order to eliminate the TL resulting from previous gamma or heavy ion irradiation.     

Detection, dosimetry and microdosimetry using high energy C beams

Samples of polyallyldiglycolcarbonate (PADC) track etch detectors (TED) were exposed to high energy ¹²C nuclei at the particle beam of the Dubna synchrophasotron. The energy of ¹²C nuclei varied between 0.1 and 1.5 GeV per amu.

At the low studied energies the linear energy transfer (LET) of these nuclei is higher than the detector threshold value. Then, the primary particle tracks are directly etched in the detector surface. The detection efficiency approaches to 100% at perpendicular incidence. Their LET has been established by means of standard authomatized procedure recently developed. The LET values found here are in good agreement with theoretical ones.

At 1.5 GeV per amu (LET 8.4 KeV μm⁻¹) the secondary particle tracks were evaluated in all the exposed detectors. The energy deposited by these particles was compared to the energy deposited through primary ionization losses. It was found out that its contribution to the total dose is relatively lower than for protons of comparable energies. In some of these samples even the tracks of the primary nuclei were observed. It follows that the detection threshold of the developed LET spectrometer should be below 10 keV μm⁻¹.     

The calibration of the solid state nuclear track detector LR 115 for radon measurements

An experimental calibration of indoor room and outdoor soil detector devices which are based on LR 115 as sensitive element has taken place at the Swedish Radiation Protection Institute in Stockholm (Sweden) in 1994 and 1996, at the Physikalisch-Technischen Bundesanstalt in Braunschweig (Germany) in 1997 and at the Umweltforschungszentrum Leipzig-Halle (Germany) in 1997. Special properties of the used solid state nuclear track detector (SSNTD) material LR 115^* have been measured to define the application of the experimental calibration.     

Energy loss dependent transversal etching rates of heavy ion tracks in plastic

By the method of electrolytical etching track etching rates V_t and corresponding transversal track etching rates V_trans of single heavy ion tracks in thin Makrofol KG foils have been measured at ion energies from 10–480 MeV/u. Makrofol KG foils of 8 μm thickness were irradiated perpendicular to the surface with ⁷⁹Au and ⁵⁴Xe ions at specific energies with energy loss values of REL=(10–90) ^*10³ MeVcm²/g at GSI Darmstadt, Germany, and Lawrence Berkeley Lab., Cal., USA. Using the electrolytical etching method by measuring the resistance of the foil during the etching process (etching conditions: 6n NaOH, room temperature and controlled 50° C) the breakthrough time and track etching rates V_t, V_trans and V_m (bulk etching rate) were analysed. Response curves (V_t/V_m)-1 as a function of Restricted Energy Loss (REL), the maximum extension of the ion induced damage perpendicular to the ion path and the dimension of the ion track core depending on the deposited energy can be estimated.     

Natural activities of U, Th and K in some Indian building materials

The activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K have been determined by gamma-ray spectrometry. The measured activity in the selected building materials ranges from (3.2 to 151.7 Bq/kg), 14 to 63.7 Bq/kg and 24.3 to 121.5 Bq/kg for ²²⁶Ra, ²³²Th and ⁴⁰K respectively. The activity concentration of ²³⁸U has been determined using fission track technique and the value ranges from 0.11 to 3.85 ppm. The concentrations for these natural radionuclides are compared with the reported data from other countries. Radium-equivalent activities (Ra_eq) are calculated for the measured samples to assess the radiation hazards arising due to the use of these materials in the construction of dwellings. All building materials have shown Ra_eq activities lower than the limit set in the Organization for Economic Cooperation and Development (OECD) report (370 Bq/kg), equivalent to external gamma dose of 1.5 mSvyr⁻¹. A good correlation has been observed between ²³⁸U and ²²⁶Ra in these materials.     

Effects of low-LET radiations on CR-39 track detector

The effects of Co-60 gamma-rays and about 25-MeV pulsed electrons have been investigated on CR-39 plastic nuclear track detector. The bulk etch rate was measured as a function of the absorbed dose under three different irradiation conditions; gamma-ray exposure in air at the dose rate of 6 kGy/h, that in vacuum and electron shower in air at 126 kGy/h on average. It was found that the bulk etch rate (V) increased nearly exponentially with the dose (D); V=V₀exp(kD), and the characteristic parameter (k) was 35, 9 and 12 MGy⁻¹, respectively. It is considered that a lack of oxygen in damage formation processes could occur momentarily during a short electron pulse, just similarly to the vacuum effect of reduction in track etch rates in outgassed CR-39 plastic.     

Study of fossil tracks due to VH and VVH groups of cosmic ray nuclei in olivine crystals from Luna-16 and Luna-24

The olivine crystals from lunar regolith samples taken by the Soviet unmanned spacecrafts Luna-16 and Luna-24 were investigated. Eleven 0.5 –1.0 mm size olivine crystals were mounted in epoxy, polished and then etched in modified WO₄ solution. The Fe-group track densities up to 10⁸ tracks.cm⁻² (Fe-group) were measured under optical microscope. The tracks of length greater than 30 microns due to Z ≥ 36 cosmic ray nuclei are counted for VVH tracks density for all the crystals. The VVH / VH track densities ratio for these lunar olivine crystals varies from 1.25×10⁻⁴ to 2×10⁻³. It corresponds to the averaged depth of these crystals in lunar soil of 2–8 cm during galactic cosmic ray exposure. Lunar crystals are well suited for VVH track studies due to a very high track density. Two crystals were annealed at 430°C for 32 hrs. This procedure eliminates iron group tracks completely and leaves etchable tracks of nuclei with Z 50 even in the olivine crystals with Fe-group tracks up to 1–2×10⁸ tracks cm⁻². We were able to measure two tracks with the length 195 and 210μm which were produced by Th---U group of Galactic cosmic ray nuclei.     

A comparative study of gamma radiation effects on track properties of different PADC detectors

Track properties of gamma-irradiated polyallyldiglycol carbonate (PADC) detectors (Homalite, Pershore and Trastrack) are studied in the dose range of 10¹–10⁶ Gy. Results show that the bulk and track etch rates of all three types of PADC detector increase at doses higher than 10⁴ Gy. This increase is more pronounced in post-gamma exposed detectors. The change in etch rate ratio (S) for post-gamma exposed PADC–Homalite detectors is more significant when compared with the other detectors. The critical angle for etching shows that etching at a low temperature of 60 °C is more effective, and the critical angle value is decreased to a significant extent for all three types of PADC detectors. Etching efficiency of post-gamma exposed samples is found to be much higher than the pre-gamma exposed sample at the highest dose of 10⁶ Gy. When compared, it was found that of the three PADC detectors, PADC–Homalite detectors seem to be more sensitive to gamma irradiation.     

Structure and luminescence of a-plane GaN on r-plane sapphire substrate modified by Si implantation

We show the structural and optical properties of non-polar a-plane GaN epitaxial films modified by Si ion implantation. Upon gradually raising Si fluences from 5×10¹³ cm^-2 to 5×10¹⁵ cm^-2, the n-type dopant concentration gradually increases from 4.6×10¹⁸ cm^-2 to 4.5×10²⁰ cm^-2, while the generated vacancy density accordingly raises from 3.7×10¹³ cm^-2 to 3.8×10¹⁵ cm^-2. Moreover, despite that the implantation enhances structural disorder, the epitaxial structure of the implanted region is still well preserved which is confirmed by Rutherford backscattering channeling spectrometry measurements. The monotonical uniaxial lattice expansion along the a direction (out-of-plane direction) is observed as a function of fluences till 1×10¹⁵ cm^-2, which ceases at the overdose of 5×10¹⁵ cm^-2 due to the partial amorphization in the surface region. Upon raising irradiation dose, a yellow emission in the as-grown sample is gradually quenched, probably due to the irradiation-induced generation of non-radiative recombination centers.     

β-SiC单晶块体拉伸变形行为分子动力学研究

用分子动力学方法研究面心立方SiC(β-SiC)、C元素置换掺杂β-SiC单晶块体体系[001]向拉伸变形行为,对体系的拉伸断裂微观机理和5×10⁸/s,1×10⁹/s,1×10¹⁰/s三种应变速率下的力学行为进行分析.发现当拉伸应变量达到某个临界值后,Si-C sp³,C-C sp³键会分别向Si-C sp²和C-C sp²弱键转化.形成一定量sp²键后,β-SiC内部出现孔洞并发生断裂.由于C-C sp³键比Si-C sp³键更易向sp²弱键转化,导致C元素掺杂会降低SiC的强度、弹性模量和拉伸断裂应变.此外还发现此三种应变速率不影响β-SiC的杨氏模量,但影响其拉伸强度.     

In0.53Ga0.47As/InP量子阱与体材料的1 MeV电子束辐照光致发光谱研究

为获得对In_0.53Ga_0.47As/InP材料在电子束辐照下的光致发光谱变化规律, 开展了1 MeV电子束辐照试验, 注量为 5×10¹²-9×10¹⁴ cm^-2. 样品选取量子阱材料和体材料, 在辐照前后, 进行了光致发光谱测试, 得到了不同结构In_0.53Ga_0.47As/InP材料在1 MeV电子束辐照下的不同变化规律; 对比分析了参数退化的物理机理. 结果显示: 试验样品的光致发光峰强度随着辐照剂量增大而显著退化. 体材料最先出现快速退化, 而五层量子阱在注量达到6×10¹⁴ cm^-2时, 就已经退化至辐照前的9%. 经分析认为原因有: 1)电子束进入样品后, 与材料晶格发生能量传递, 破坏晶格完整性, 致使产生的激子数量减少, 光致发光强度降低; 电子束辐照在材料中引入缺陷, 增加了非辐射复合中心密度, 导致载流子迁移率降低. 2)量子阱的二维限制作用使载流子运动受限, 从而能够降低载流子与非辐射复合中心的复合概率; 敏感区域截面积相同条件下, 体材料比量子阱材料辐射损伤更为严重. 3)量子阱的层数越多, 则异质结界面数越多, 相应的产生的界面缺陷数量也随之增多, 辐射损伤越严重.     

Search for the ternary cluster decay of Cm isotopes using SSNTD

Investigation of the ²⁴⁶Cm spontaneous ternary decay into nearly equal clusters using polyethyleneterephthalate track detectors has been carried out. The possibility of ternary cluster decay has been found out to be of less than 10⁻⁶ with respect to all fission modes of ²⁴⁶Cm.     

A new method to measure track density and to differentiate nuclear tracks in CR-39 detectors

An alternative method to count and differentiate nuclear tracks in SSNTD is described. The method is based on the analysis of Fraunhofer diffraction pattern of coherent light produced by tracks of an etched SSNT detector. The diffraction pattern was also simulated by applying computational Fourier Optics. The comparison between results obtained by simulation and by the theoretical model gave satisfactory concordance. The proposed method is capable of differentiating tracks in CR-39 by their diameter and energies. The diameter resolution ranged between 8% and 25%, while the counting error was less than 15%. The discriminating ability to distinguish genuine etched tracks from defects and background anomalies is demonstrated. The incidence angle did not influence significantly the total count and the track parameter measuring capability. Errors due to track overlapping are only significant for track densities higher than 3×10⁵ cm⁻².     

ZBLAN玻璃中Pr3+掺杂离子3P0和1D2能级的发光特性和寿命

对ZBLAN氟锆酸盐玻璃中Pr³⁺掺杂离子³P₀和¹D₂能级的寿命和发光特性进行了较详细的光谱学研究。首先测量了两种掺杂浓度(质量分数分别为1×10^-3,5×10^-3)的Pr³⁺:ZBLAN玻璃的吸收光谱,然后运用时间分辨激光光谱技术测量了³P₀和¹D₂能级在激光单光子共振激发下的荧光发射谱和能级寿命。将不同荧光发射谱带的强度和文献报道的Judd Ofelt理论计算辐射跃迁几率数值做了比较分析,证明了文献中理论计算结果的可靠性。由于浓度猝灭效应,在相同的激发条件下,掺杂浓度为1×10^-3样品的荧光发射强度明显大于5×10^-3样品的荧光发射强度。但是从我们的测量结果看,掺杂浓度对³P₀和¹D₂ 的能级寿命值无显著影响。掺杂浓度为1×10^-3时,Pr³⁺离子³P₀和¹D₂能级的寿命值分别为46,322μ_s。