Double Escape Peak Research Articles

Experimental study of the response characteristics of large Ge(Li) detectors

The relative efficiencies of a cylindrical Ge(Li) detector having an estimated active volume of 77 cm 3 were measured in the full-energy, single-escape and double-escape peaks produced by γ rays of energies 2–12 MeV. These relative efficiencies at 2–10 MeV are expressed theoretically in terms of the absolute efficiencies calculated for all interactions except coherent scattering, the probability that a 0.511 MeV annihilation photon escapes from the active volume of the crystal, and an empirical reduction factor which is energy dependent. The escape probability is constant at 0.676±0.006. The γ-ray absorption coefficient for all interactions producing the full-energy peak is derived by comparison with the known pair-production coefficient and tabulated for γ rays of energies 2–10 MeV. The above Ge(Li) detector and a second one having an estimated active volume of 114 cm 3 were scanned horizontally with a fine pencil of γ rays from a 10 mg radium needle. The relative angular efficiencies measured in the full-energy peaks at 1120, 1764, 2204 and 2447 keV were used to evaluate the angular attenuation factor Q k for k = 1, 2, 4 and 6. The experimental and calculated factors agree if the effective outer radius of the detector is taken to be 87% of the nominal value. Radiography was found useful in checking the dimensions and axial position of one of the detectors in its aluminum housing.

An interesting form of high‐energy gamma‐ray–detector interaction and its effect on spectral peak centroid determination

The pulse-height continuum produced in a radiation detector following the interaction of photons by pair-production is discussed. The event when one annihilation photon escapes from the detector without interaction while the other escapes after one Compton scattering interaction produces a characteristic component in the pulse-height spectrum. The significance of this component in influencing the accurate determination of the centroid of the double-escape peak is examined and shown to be negligibly small.

The determination of fluorine in bones by non-destructive neutron activation analysis using 11.2 sec20F

A fast non-destructive determination of fluorine in bone samples by thermal neutron activation analysis using19F(n, γ)20F reaction was developed. About 0.1–1 g samples is irradiated for 15 sec in TRIGA MARK II reactor at a thermal neutron flux of 5·1011 n·cm−2·sec−1. After 15–25 sec cooling, the 1633 KeV20F activity (T=11.2 sec) is counted for 15 sec with a Ge(Li) spectrometer. A standard sample is prepared by mixing CaF2 and CaCO3 powders. The interference from23Na(n, α)20F is corrected by employing24Na 2754 KeV double escape peak activities in samples and the20F/24Na peak area ratio observed previously for pure Na2CO3 powder. The precision is 7% for a bone sample containing 1020 ppm F and the sensitivity is about 10 ppm F.

Determination of detector efficiencies for gamma ray energies up to 12 MeV II. Monte Carlo calculation

By means of the Monte Carlo method the absolute intrinsic double escape peak efficiencies of a planar Ge(Li) detector and a cylindrical detector with an active volume of 26 cm 3 for gamma ray energies up to 12 MeV and the absolute full energy peak efficiencies of a cylindrical detector with an active volume of 42 cm 3 for gamma ray energies up to 3 MeV have been calculated. The evaluated efficiencies were compared with experimental values known from literature. An overall very satisfactory agreement was found taking multiple scattering of secondary particles produced in gamma ray interaction events, the creation of bremsstrahlung and the theoretical energy range relations into account avoiding crude approximations in the treatment of the usual interaction processes such as the photoelectric effect, the Compton scattering effect, the pair production effect, and the annihilation of the positron. The production of triplets and the annihilation in flight had no remarkable influence on the calculated efficiencies in the gamma ray energy range considered.

Fast calibration of large Si(Li) electron detectors from 511.0 to 4564.0 keV using double-escape peaks and compton edges from 66Ga

Double-escape peaks and Compton edges from the γ-rays of 9.5-h 66Ga have been used to allow fast, precise calibration of Si(Li) electron detectors. They allow a direct calibration up to 4564.0 keV, and the calibration curve is linear enough that it can probably be extrapolated safely to even higher energies.

Efficiency determination for some standard type Ge(Li) detectors for gamma-rays in the energy range from 0.04 to 11 MeV

Absolute full energy and double escape peak efficiencies of 38 and 31 cm 3 true coaxial and of 5.8 cm 3 planar Ge(Li) detectors have been determined for γ-rays from 0.04 to 11 MeV in an arrangement typical for γγ-coincidence experiments with thermal neutron capture γ-rays from an external target. The results are compared with theoretical predictions. Experimental results for the ratios of double escape, single escape and full energy peak areas against the γ-ray energy are presented. The measurements have been performed with the γ-rays from the radioactive sources 153Gd, 169Yb and 154Eu and from the thermal neutron capture in 1H, 12C, 14N, 39K and 53Cr. To check the efficiency curves obtained in the present experiment, the relative intensities of some γ-ray transitions in the reactions 39K (n, γ) 40K and 45Sc(n, γ) 46Sc are compared with the results of other authors.

Monte Carlo calculations of relative efficiencies of Ge(Li) detectors

Abstract By means of a Monte Carlo computer program the relative full energy peak efficiency and relative double escape peak efficiency for gamma rays of a cylindrical Ge(Li) detector with volume of 50 m 3 , a rectangular Ge(Li) detector with volume of 30 cm 3 and a trapezoidal Ge(Li) detector with volume of 50 cm 3 has been calculated. The gamma source was assumed to be a point placed 10 cm from the detector. The calculated efficiency curves for the cylindrical detector were compared with the curves experimentally obtained by other authors. The agreement was found to be within quoted errors. The calculated efficiency curves of all three detectors have been used to determine the gamma ray relative intensities in the decays of 22 Na, 60 Co, 56 Co and 226 Ra. It has been found that the relative intensity values of gamma rays for the above sources can be reproduced within 5%.

Final Results on the Nature of the Pulse-Height Spectrum from Semiconductor Detectors

An analysis of the shape of the pulse-height spectrum is carried out. The se quence of the physical processes which participate in shaping signals in a detector is taken into consideration. The main attention is paid to the charge collection process and to the spread in the amplitude Co(q) due to incomplete collection. A generalized analytical expression is obtained for the quantity q and the function Co(q) for detectors of the most widely used configurations (planar, coaxial, spherical). An analysis is given of the difference in two conventional approaches 6,8 to the problem of calculating the total pulse-height spectrum shape. It is noted that under ordinary conditions of gamma spectrometry the spectrum undergoes essential symmetrization due to multiple scattering (Compton scattering with subsequent photoabsorption). This brings the results of calculations outlined in refs. 6,8 closer to one another. However, in a single-step interaction (resulting, for instance, in the double escape peak) simple summation of dispersions for the spectrum components in 6 would lead to considerable errors. The scheme outlined in 8,12 is shown to permit using a pulse-heght analyzer in calculations of the spectrum shape. Presented in the Appendix are the results of an experiment showing that the signal across the detector electrodes is connected with potential differences traversed: Δq~Δv/v This relationship was used to calculate the charge q and the spectrum Co(q).

Relative efficiency of Ge(Li) gamma ray detectors from 0.5 to 12 MeV

The double-escape peak efficiency for several Ge(Li) detectors has been measured between 1.6 and 11.6 MeV using radioactive sources and gamma-ray cascades following proton capture reactions. These measurements were made relative to the full-energy peak efficiency which was obtained between 0.5 and 3.25 MeV. The data were fitted by empirical functions and the overall accuracy of the relative efficiency was between 3% and 5% for the full-energy peak and 6% for the double-escape peak. Several examples of branching ratio measurements are presented and the relative intensity of weak gamma-ray transitions following 56Co beta decay has been determined.

Analytical sensitivities and energies of thermal neutron capture gamma rays II

A table of the analytical sensitivities of the principal lines in the thermal neutron capture gamma-ray spectrum from 0 to 3 MeV has been compiled for most of the elements. A tabulation of the full-energy, single-escape, and double-escape peaks has also been made according to energy. The tables are useful for spectral interpretation and calibration.

Separately Housed Ge(Li) Detectors Operating in Parallel and as a Summing Compton Spectrometer

Coaxial as well as planar Ge(Li) γ-ray detectors housed in separate cryostats have been operated in parallel and as a summing Compton spectrometer. The linear signals from two detectors were direct coupled into a single charge-sensitive preamplifier, thus eliminating the need for matching electronic gain and linearity of two separate channels. The full-energy peak efficiency of two parallel coaxial detectors is nearly equal to the sum of the efficiencies of the individual detectors. Similar increase in efficiency was also observed for double-escape peaks when two small planar detectors were paralleled. The FWHM of the 1.33 MeV 60Co line was 2.94 keV in the parallel mode as compared with 2.23 and 2.38 keV for the constituent 22cc coaxial detectors. When the latter detectors were used as a summing Compton spectrometer, the Compton edge was completely eliminated. The peak-to-Compton ratio in a 137Cs spectrum was 25:1 in the singles mode and 125:1 in the coincidence mode. However, in the complex Th C" spectrum the Compton spectrometer was not very effective in resolving a weak low energy line from the continuum of several intense γ-rays of higher energy. Thus, one large, high resolution detector or several in parallel is not only simpler and more efficient, but may also be more effective than the summing Compton spectrometer.

Analytical sensitivities and energies of thermal-neutron-capture gamma rays

A table of the analytical sensitivities of the principal lines in the thermal-neutron-capture gamma ray spectrum has been compiled for most of the elements. In addition a second table of the full-energy, single-escape, and double-escape peaks has been compiled according to energy for all significant lines above 3 MeV. Lines that contrast well with adjacent lines are noted as prominent. The tables are useful for spectral interpretation and calibration.

Etude des caracteristiques d'un detecteur Ge(Li) de 120 cm 3 et comparaison avec des detecteurs plus petits

We show some gamma ray energy spectra obtained with a 120 cm 3 active volume Ge(Li) detector. Energy resolution and counting efficiency curves versus energy for total absorption peak and double-escape peak are drawn. In addition, a comparison between characteristics is made with smaller detectors and some conclusions are deduced for great detector volume uses.

Monte Carlo calculations of gamma ray response characteristics of cylindrical Ge(Li) detectors

Monte Carlo calculations have been made to compute the full energy peak efficiencies and the double escape peak efficiencies for right circular cylindrical Ge(Li) detectors for point gamma-ray sources. The full energy peak efficiencies are calculated in the gamma-ray energy range from 100 keV to 2.5 MeV and the double escape peak efficiencies are calculated in the range from 1.7 MeV to 4.0 MeV. All calculations have been made for source to detector distance of 5 cm. Various detector sizes have been considered with surface area varying from 2.5 cm 2 to 8.0 cm 2 and the thickness varying from 2.0 to 10.0 mm. The results are compared with the calculations of earlier workers and also with some of the experimental investigations. A code “MONTCARL” has been developed for this purpose in FORTRAN language for the CDC 3600 computer. The same code can be used to calculate the response function of the detector to gamma-rays. One such calculation has been made for monoenergetic gamma-rays and it compares very well with the experimental results.

Si(Li) Coaxial Detectors

Lithium drifted silicon coaxial detectors have been fabricated from 19 mm diameter material with volumes up to 5 cm3. Essentially single line response, the double escape peak, is observed for gamma rays interacting by pair production. Resolutions (FWHM) of 5 keV and approximately 10 keV have been observed for incident gamma rays of 2.62 and 7.64 MeV. Slices, 2 to 3 mm thick, have been taken from such coaxial detectors and used as low capacity, high resolution X-ray detectors. A FWHM of 300 eV has been obtained at 6.4 keV using a detector with an active area slightly greater than 200 mm2.

Calculated relative efficiency for coaxial and planar Ge(Li) detectors

With the aid of a Monte Carlo computer program, the relative efficiency to monoenergetic γ-rays of coaxial and planar Ge(Li) detectors has been calculated. Considering a point source, placed on the extended axis of the detector, at a fixed distance, radiations of different energies are generated within the solid angle subtended by the crystal. The gamma-rays impinging on the detector are followed until a fraction whatsoever of their energy escapes from the active region of the crystal. If the γ-ray releases all its energy in the active region, a photopeak is counted and then its efficiency is calculated. At the end of the program the efficiency for the double-escape peak is also obtained. Following this procedure the photopeak efficiency was calculated for three Ge(Li) detectors, having volumes of: 2.0 cm 3 (planar), 5.3 cm 3 and 30 cm 3 (coaxial) from ≈ 100 keV up to 4000 keV. By means of these calculated efficiency curves, the well measured gamma-ray relative intensities in sources as 22Na, 88Y, 207Bi, 133Ba and ThC″ could be reproduced to within 1%–3%.

Line shape analysis of spectra obtained with Ge(Li) detectors

Line shape analysis techniques have been applied to gamma ray spectra obtained with a Ge(Li) detector. It was found that least-squares fitting procedures enabled more accurate and rapid extraction of relative intensities than was possible from the areas of the full energy or double escape peaks.

A pair and escape-suppressed spectrometer using Ge(Li) and NaI(Tl) detectors designed for accelerator experiments

A high resolution gamma-ray spectrometer consisting of a 40 cm 3 Ge(Li) detector located at the common centre of two semi-annular NaI(Tl) detectors has been assembled for use in particle accelerator experiments. The three detectors are operated in a triple coincidence and an anti-coincidence mode, simultaneously yielding pair and escape-suppressed spectra in which background and the multiplicity of gamma-ray peaks are reduced. In the pair spectrometer mode the background is reduced considerably relative to the double escape peak e.g. a factor of 20 for a 3.85 MeV gamma ray, and in the escape-suppressed mode by a factor of three to four. The performance of the spectrometer is demonstrated using a Pu- 10B gamma-ray source.

Calculation of double escape peak angular correlation attenuation coefficients for square Ge(Li) gamma detectors

Angular correlation attenuation coefficients for the double escape peak are calculated for nine Ge(Li) detectors, by the use of a simple Monte Carlo technique. The calculations are valid for photon energies between 1.5–6 MeV. The statistical model used is empirically adjusted to fit the observed intrinsic efficiency for energies from 6 to 10 MeV. This gives an approximation to the attenuation coefficients for these energies. The coefficients are compared to the “any interaction” attenuation coefficients.

Studies of Detection Efficiencies and Operating Characteristics of Ge(Li) Detectors

The rapidly expanding use of Ge(Li) detectors in all fields of gamma-ray spectroscopy has led to a need for information from which experimenters may effectively select the best detector for their special needs. This is necessitated because of the large variety of detector sizes and configurations which are currently available and the lack of standardization of sizes. Several criteria should be considered in the selection of a detector for a specific application. Among these are: 1) detection efficiency, 2) charge collection efficiency, which gives rise to abnormal peak broadening as a function of gamma-ray energy as well as to asymmetric peak shapes, 3) timing characteristics for coincidence applications, and 4) energy resolution which is one of the measures of the ability to identify a weak gamma-ray in the presence of strong higher-energy gamma rays. We have measured these characteristics for several planar and open-ended and five-sided coaxial detectors. Absolute full-energy peak, double-escape peak and single-escape peak efficiencies were obtained for gamma-ray energies from 30 keV to 11 MeV. For planar devices, the effects of thickness and cross-sectional area of the depleted volume have been investigated. Some studies have been made of the problems involved in measuring gamma-ray intensities. These include measurements of the counter "edge effects" by a comparison of the efficiency curves obtained at different source-detector distances. Other studies involve random and real coincidence summing effects which are countingrate and geometry-dependent effects.