Ray Emission Research Articles

Reconciling surrogate-reaction probabilities and neutron-induced cross sections

Since its inception, the so-called surrogate-reaction method (SRM) has motivated the development and improvement of theories in connection to direct reactions. This paper reassesses some of the developments carried out in previous decades to deal with the representation of direct reaction probability data. It is believed that the experimental probabilities assimilation in the neutron cross section evaluation process can be better estimated using tools resulting from the efforts made over the years. This paper provides a new perspective on this issue both in terms of fission and $\ensuremath{\gamma}$-ray emission probabilities. In addition to the ``natural'' assimilation path that considers analyzing probabilities jointly with cross sections to extract nuclei structural properties, this article puts forward a prescription to convert, with a good level of confidence, measured direct-reaction induced probabilities to pseudoexperimental neutron-induced cross sections. This approach is named after the SRM as extended SRM (ESRM).

Investigating the Fermi Large Area Telescope sensitivity of detecting the characteristics of the Galactic center excess

The center of the Milky Way is offering one of the most striking mysteries in astroparticle physics. An excess of $\ensuremath{\gamma}$ rays (GCE) has been measured by several groups in the data collected by the Fermi Large Area Telescope (LAT) toward the Galactic center region. The spectrum and spatial morphology of the GCE have been claimed by some groups to be compatible with a signal from the galactic halo of dark matter (DM). Instead, other analyses have demonstrated that the GCE properties, e.g., its energy spectrum, highly depend on the choice of the galactic interstellar emission (IEM) model source catalogs and analysis techniques. In this paper, we investigate the sensitivity of Fermi LAT to detect the characteristics of the GCE. In particular, we simulate the GCE as given by DM, and we verify that, with a perfect knowledge of the background components, its energy spectrum, position, spatial morphology, and symmetry is properly measured. We also inspect two more realist cases for which there are imperfections in the IEM model. In the first, we have an unmodeled $\ensuremath{\gamma}$-ray source, constituted by the low-latitude component of the Fermi bubbles. In the second, we simulate the data with one IEM template and analyze the data with another. We verify that a mismodeling of the IEM introduces a systematics of about 10--15% in the GCE energy spectrum between 1--10 GeV and about 5% in the value of the slope for a Navarro-Frenk-White DM density profile, which is used to fit the GCE spatial morphology. Finally, we show how the GCE would be detected in case of alternative processes such as $\ensuremath{\gamma}$-ray emission from a bulge population of pulsars or from electrons and positrons or protons injected from the Galactic center. We demonstrate that for each of these cases, there is a distinctive smoking gun signature that would help to identify the real mechanism behind the origin of the GCE.

Impacts of gamma ray emissions of materials containing 232U on safety, security, and safeguards

The introduction of 232U to uranium materials is a possible technique to reduce the material attractive of the material for weapons purposes. In order to implement 232U in civilian nuclear fuel cycles, its impacts on safety, security, and safeguards must be considered. This study will utilize an initial mixture of 0.03 at.% 232U with natural uranium, enriched to approximately 3, 5, 20, and 90 at.% 235U. The mixture with natural uranium, 3, 5, and 20 at.% 235U represent uranium isotopic vectors that would be commonly found within civilian nuclear fuel cycles. The 90 at.% 235U isotopic vector represents weapons grade uranium. The security and safeguards results from each isotopic vector will be compared to results from a corresponding isotopic vector that does not contain 232U, but has the same 235U/238U ratio. Considering 232U’s daughter products contribute significantly to the gamma ray emissions over time, the isotopic vectors will be evaluated at three time intervals: fresh, 0.5 year, and 10 years. When compared to standards put in place by U.S. Nuclear Regulatory Commission, the dose rates produced by the uranium isotopic vectors containing 232U indicate the need for additional safety protocol to shield workers from this increased dose rate. These gamma ray emissions produce a significant increase in detectability in the weapons grade uranium isotopic vector with 232U even in the presence of lead shielding. A common safeguards technique used to verify 235U enrichment was shown to be useless in the presence of 232U and its daughter products. This would require the use of a different technique in order to verify the 235U enrichment.

Preliminary investigation of the shutdown radioactivity for EAST based on gamma ray spectrometry

• The HPGe spectrometer system was placed inside the vacuum vessel to measure the characteristic γ-rays emission from radionuclides after EAST shutdown. • The identification and pathway analysis of radionuclides were discussed based on structural materials and radiation sources of EAST. • Activity of radionuclides and time evolution of dose rate, preliminary activation results, were discussed. Experimental Advanced Superconducting Tokamak (EAST) is capable of operating with long-pulse deuterium plasma, which will generate a lot of radiation such as fusion neutrons, gamma rays and hard X-rays. Since the structure material of the device was activated, work around and inside the tokamak would be limited from a radiation protection point of view after shutdown. Identification of radionuclides and evaluation of dose rate inside the vacuum vessel are needed to develop a guiding principle for radiation protection during maintenance. In this paper, a gamma ray spectrometer based on high purity germanium (HPGe) detector and digitizer was placed inside the vacuum vessel to measure the characteristic gamma ray emission from radionuclides. 7 Be, 46 Sc, 51 Cr, 54 Mn, 58 Co, 60 Co, 59 Fe, 88 Y, 91 Y, 91m Nb, 99 Mo, 124 Sb and 110m Ag produced by photonuclear reactions and neutron-induced reactions were identified. The activities of radionuclides was calculated respectively with the uncertainty not exceed 6.05 %. The dose rate for 10 days after shutdown was mainly contributed by 60 Co, 58 Co, 54 Mn, 110m Ag, 59 Fe, 51 Cr, 88 Y, 99 Mo, 7 Be and 124 Sb, total dose rate were 3.62 × 10 −9 Sv/h(Port E) and 3.40 × 10 −9 Sv/h(Port I).

The Correlations between Radio and [formula omitted]-ray Emissions in Multibands for Fermi Blazars

A sample including 935 blazars with 415 Flat Spectrum Radio Quasars (FSRQs) and 520 BL Lac objects (BL Lacs) (233 HBLs (High synchrotron peak frequency BL Lacs), 144 IBLs (Intermediate synchrotron peak frequency BL Lacs), and 143 LBLs (Low synchrotron peak frequency BL Lacs)) is selected in Fermi 3FGL catalogue. The correlations between two flux densities of the radio 1.4GHz and γ-ray at 0.1, 0.3, 1, 3, and 10GeV are investigated for the samples of all blazars, FSRQs, BL Lacs, and subclasses of HBLs and LBLs, respectively. Results show that there are strong positive correlations between the two flux densities, with the correlation coefficients r = 0.48∼0.81, and the chance probabilities p all less than 10−4. The correlation coefficient has different trends with the change of γ-ray emission frequency for different blazar samples, and the average correlation coefficients for all the samples at five respective bands decrease with the increase of γ-ray frequency. Therefore, with the increase of γ-ray frequency, the dominant mechanism of γ-ray radiation of blazar is changing, and at the same frequency the dominant mechanism is different for different samples. The γ-ray emission of HBLs may be mainly dominated by the synchrotron self-Compton, while the other mechanisms are responsible for the γ-ray emissions of LBLs. The origin of γ-ray in FSRQs is more complex than that of BL Lacs.

The prolonged gamma ray enhancement and the short radiation burst events observed in thunderstorms at Tien Shan

We report the observation results of the hard radiation flashes which accompanied the lightning discharges above the mountains of Northern Tien Shan. Time series of the counting rate intensity, numerical estimations of absolute flux, and energy distribution of accelerated electrons and of (20−2000) keV gamma rays were obtained at the height of 3700 m a. s. l., immediately within thunderclouds, and in closest vicinity (≲100 m) to discharge region. Two different kinds of radiation emission events are presented here: a relatively prolonged rise of gamma ray intensity with minute-scale duration (the thunderstorm ground enhancement, TGE) which has preceded a negative field variation, and a short sub-millisecond radiation burst, which accompanied a close lightning discharge in thundercloud. It was revealed also an indication to positron generation in thunderclouds at the time of gamma ray emission, as well as modulation of the neutron counting rate in Tien Shan neutron monitor which was operating at a (1.5–2) km order distance from the region of lightning development.

γ-ray emission in radio galaxies under the VLBI scope

Aims. This is the second paper in our series studying the evolution of parsec-scale radio emission in radio galaxies in the Southern Hemisphere. Following our study of the radio and high-energy properties of γ-ray-emitting sources, here we investigate the kinematic and spectral properties of the parsec-scale jets of radio galaxies that have not yet been detected by the Fermi Large Area Telescope (Fermi-LAT) instrument on board NASA’s Fermi Gamma-ray Space Telescope. For many sources, these results represent the first milliarcsecond resolution information in the literature. These studies were conducted within the framework of the Tracking Active Nuclei with Austral Milliarcsecond Interferometry (TANAMI) monitoring program and in the context of high-energy γ-ray observations from Fermi-LAT. Methods. We took advantage of the regular 8.4 GHz and 22.3 GHz Very Long Baseline Interferometry (VLBI) observations provided by the TANAMI monitoring program, and explored the kinematic properties of six γ-ray-faint radio galaxies. We complemented this with ∼8.5 years of Fermi-LAT data, deriving updated upper limits on the γ-ray emission from this subsample of TANAMI radio galaxies. We included publicly available VLBI kinematics of γ-ray-quiet radio galaxies monitored by the MOJAVE program and performed a consistent Fermi-LAT analysis. We combined these results with those from our previous paper to construct the largest sample of radio galaxies with combined VLBI and γ-ray measurements to date. The connection between parsec-scale jet emission and high-energy properties in the misaligned jets of radio galaxies was explored. Results. For the first time, we report evidence of superluminal motion up to βapp = 3.6 in the jet of the γ-ray-faint radio galaxy PKS 2153−69. We find a clear trend of higher apparent speed as a function of distance from the jet core, which indicates that the jet is still being accelerated on scales of tens of parsecs, or ∼105 Rs, corresponding to the end of the collimation and acceleration zone in nearby radio galaxies. We find evidence of subluminal apparent motion in the jets of PKS 1258−321 and IC 4296, and no measurable apparent motion for PKS 1549−79, PKS 1733−565, and PKS 2027−308. For all these sources, TANAMI provides the first multi-epoch kinematic analysis on parsec scales. We then compare the VLBI properties of γ-ray-detected and undetected radio galaxies, and find that the two populations show a significantly different distribution of median core flux density, and, possibly, of median core brightness temperature. In terms of correlation between VLBI and γ-ray properties, we find a significant correlation between median core flux density and γ-ray flux, but no correlation with typical Doppler boosting indicators such as median core brightness temperature and core dominance. Conclusions. Our study suggests that high-energy emission from radio galaxies is related to parsec-scale radio emission from the inner jet, but is not driven by Doppler boosting effects, in contrast to the situation in their blazar counterparts. This implies that γ-ray loudness does not necessarily reflect a higher prevalence of boosting effects.

First Analysis of Earth's Stratospheric γ‐Ray Emission in Geographical Coordinates With Fermi LAT

AbstractFrom low‐Earth orbit, the Earth is an extremely bright γ‐ray source compared to distant astrophysical objects. The high‐energy γ‐ray emission, from the Earth's upper atmosphere, is produced by the interactions between cosmic rays (CRs) and air molecules. As CRs are predominantly charged particles, the geomagnetic field acts as a spectrometer, forbidding CRs below a certain energy to arrive and interact with the atmosphere at a given geomagnetic location from a given direction. We use photon data between 0.2 and 20 GeV as observed by the Fermi Large Area Telescope (LAT) to create, for the first time, stratospheric γ‐ray emission maps of the Earth in the geographical coordinate system, for different viewing directions from LAT altitude. These maps confirm a strong association between measured γ‐ray fluxes and geomagnetic cutoffs for CRs. We also report the spectrum of the Earth's stratospheric γ‐ray emission in cardinal directions and use the geomagnetic spectrometer effect to infer the atmosphere's yield function of γ rays per incoming CR proton as a function of CR rigidity at the altitude of the Fermi spacecraft. This work is relevant to the study of near‐Earth radiation exposure for astronauts and spacecraft, modeling the geomagnetic field, and CR‐air interactions.

Quantum mass of an electron in the hydrogen atom derived from the relativistically modified Schrödinger equation

In Bohr's theory, the photon emission or absorption by the hydrogen atom is expressed by the frequency condition. In the hydrogen atom, the eigenvalue of energy derived from the relativistically modified Schrödinger equation contains the quantum mass of an electron. The frequency condition is explained using this mass. The electromagnetic wave (e.g., X rays) emission from the highly accelerated free electron was thus predicted from this mass.

Feasibility study of a shielding-independent radiation portal monitor system for revealing 241-Am orphan sources in radiometric surveillance of scrap metal

Orphan radioactive sources inadvertently melted in steel plants are a health and economical hazard. Thus, regulation authorities impose strict control measurements to be performed at these plants. However, a recent series of incidents related to the melting of {}^{241}hbox {Am} sources demonstrated the inadequacy of even state of the art systems in revealing this low-energy gamma ray emitter up to activities of the order of hundreds GBq. However, {}^{241}hbox {Am} sources are characterized by a weak neutron emission which can be detected by proper monitor systems. This work provides a feasibility study of a shielding-independent neutron-sensitive portal monitor system to be installed in steel plants. Measurements were performed both with a certified {}^{241}hbox {Am} shielded source and at the weighing area of a steel plant, to assess both the detector performance and the practical feasibility of the system implementation in work-place fields. A risk evaluation analysis demonstrated that with the current control systems {}^{241}hbox {Am} sources up to 185 GBq could pass undetected with a potential exposure for workers equal to 1.7 mSv, consistent with recent accidents reports. The proposed technique could reveal {}^{241}hbox {Am} activities down to 2 GBq, decreasing the worker exposure down to 24 upmu hbox {Sv}, without interfering with the standard plant operations.

Side-chain fluorotelomer-based polymers in children car seats.

Fabric and foam samples from popular children car seats marketed in the United States during 2018 were tested for fluorine content by particle-included gamma ray emission spectroscopy (PIGE, n=93) and X-ray photoelectron spectroscopy (XPS, n=36), as well as for per- and polyfluoroalkyl substances (PFAS) by liquid and gas chromatography mass spectrometry (LC/MS and GC/MS, n=36). PFAS were detected in 97% of the car seat samples analyzed with MS, with total concentrations of 43 PFAS (∑PFAS) up to 268ng/g. Fabric samples generally had greater ∑PFAS levels than foam and laminated composites of foam and fabric. The three fabric samples with the highest total fluorine content as represented by the highest PIGE signal were also subjected to ultraviolet (UV) irradiation and the total oxidizable precursor (TOP) assay. Results from these treatments, as well as the much higher organofluorine levels measured by PIGE compared to LC/MS and GC/MS, suggested the presence of side-chain fluorotelomer-based polymers (FTPs), which have the potential to readily degrade into perfluoroalkyl acids (PFAAs) under UV light. Furthermore, fluorotelomer (meth)acrylates were found to be indicators for the presence of (meth)acrylate-linked FTPs in consumer products, and thus confirmed that at least half of the tested car seats had FTP-treated fabrics. Finally, extraction of selected samples with synthetic sweat showed that ionic PFAS, particularly those with fluorinated carbons ≤8, can migrate from fabric to sweat, suggesting a potential dermal route of exposure.

Two-stage γ ray emission via ultrahigh intensity laser pulse interaction with a laser wakefield accelerated electron beam

In this study, we investigate the generation of twin γ ray beams in the collision of an ultrahigh intensity laser pulse with a laser wakefield accelerated electron beam using a particle-in-cell simulation. We consider the composed target of a homogeneous underdense preplasma in front of an ultrathin solid foil. The electrons in the preplasma are trapped and accelerated by the wakefield. When the laser pulse is reflected by the thin solid foil, the wakefield accelerated electrons continue to move forward and pass through the foil almost without influence from the reflected laser pulse or foil. Consequently, two groups of γ ray flashes, with tunable time delay and energy, are generated by the wakefield accelerated electron beam interacting with the reflected laser pulse from the foil as well as another counter-propagating petawatt laser pulse behind the foil. Additionally, we study the dependence of the γ photon emission on the preplasma densities, driving laser polarization, and solid foil.

100 TeV diffuse $$\gamma $$-rays observation by YangBaJing Hybrid Array

Cosmic rays up to PeV energy are believed to be generated in our galaxy. $$\gamma $$ -rays at 100 TeV energies are important probe to identify PeVtrons. Besides, $$\gamma $$ -rays contain additional information on the propagation of the galactic cosmic rays. Diffuse $$\gamma $$ -rays originating from the Galactic plane from 20 to 200 TeV region are studied with early 160-day data collected by YangBaJing Hybrid Array (YBJ-HA), which is a hybrid array and consists of scintillation detectors and underground muon detectors. All-distance equi-zenith angle and time-swapping methods are used to analyze the number of $$\gamma $$ -rays excess. Because of no significant observation on $$\gamma $$ -ray emission, Helene’s method is adopted to derive the $$90\%$$ confidence level upper limits on the flux of diffuse galactic $$\gamma $$ -rays. And the limit to the ratio of the flux of $$\gamma $$ -rays to CRs is obtained at the order of $$10 ^{-4}$$ , which is the current best result at these energies.

Solar Gamma-Ray Flares and Activity Complexes

Strong flare events with an extended phase of gamma ray emission from the catalog by Share et al. have been analyzed in order to explain the characteristics of the impulsive and extended phases of such flares and reveal their connection with activity complexes and coronal holes. It is shown that 74% of such events are closely related to activity complexes. It has been qualitatively demonstrated that the particle acceleration processes during the flare development are associated with the changes in the magnetic topology of the flare region and the evolution of the coronal mass ejection. The possible relation of coronal holes to activity complexes and the role of “exchange” reconnection in these processes are discussed.

Contribution of pulsars to cosmic-ray positrons in light of recent observation of inverse-Compton halos

The hypothesis that pulsar wind nebulae (PWNe) can significantly contribute to the excess of the positron ($e^+$) cosmic-ray flux has been consolidated after the observation of a $\gamma$-ray emission at TeV energies of a few degree size around Geminga and Monogem PWNe, and at GeV energies for Geminga at a much larger extension. The $\gamma$-ray halos around these PWNe are interpreted as due to electrons ($e^-$) and $e^+$ accelerated and escaped by their PWNe, and inverse Compton scattering low-energy photons of the interstellar radiation fields. The extension of these halos suggests that the diffusion around these PWNe is suppressed by two orders of magnitude with respect to the average in the Galaxy. We implement a two-zone diffusion model for the propagation of $e^+$ accelerated by the Galactic population of PWNe. We consider pulsars from the ATNF catalog and build up simulations of the PWN Galactic population. In both scenarios, we find that within a two-zone diffusion model, the total contribution from PWNe and secondary $e^+$ is at the level of AMS-02 data, for an efficiency of conversion of the pulsar spin down energy in $e^\pm$ of $\eta\sim0.1$. For the simulated PWNe, a $1\sigma$ uncertainty band is determined, which is of at least one order of magnitude from 10 GeV up to few TeV. The hint for a decreasing $e^+$ flux at TeV energies is found, even if it is strongly connected to the chosen value of the radius of the low diffusion bubble around each source.

Gamma-ray counterparts of 2WHSP high-synchrotron-peaked BL Lac objects as possible signatures of ultra-high-energy cosmic ray emission

ABSTRACT We present a search for high-energy γ-ray emission from 566 Active Galactic Nuclei at redshift z > 0.2, from the 2WHSP catalogue of high-synchrotron peaked BL Lac objects with 8 yr of Fermi-LAT data. We focus on a redshift range where electromagnetic cascade emission induced by ultra-high-energy cosmic rays can be distinguished from leptonic emission based on the spectral properties of the sources. Our analysis leads to the detection of 160 sources above ≈5σ (TS ≥25) in the 1–300 GeV energy range. By discriminating significant sources based on their γ-ray fluxes, variability properties, and photon index in the Fermi-LAT energy range, and modelling the expected hadronic signal in the TeV regime, we select a list of promising sources as potential candidate ultra-high-energy cosmic ray emitters for follow-up observations by Imaging Atmospheric Cherenkov Telescopes.

Dense high-energy γ -rays emission by ultra-intense laser interacting with a concave target

We propose to use a 10 petawatt (PW) laser irradiating onto a target with a concave surface, which can focus the laser beam and attain a more intense laser field, so as to increase both the yield and mean energy of emitted γ-rays. 2D particle-in-cell simulation results show that the peak electric field after the reflection of the laser from the target in this new scheme can reach ∼1.8 times as high as that in the plane target case. Such an enhanced laser field enables an increase in the number of hot electrons in front of the incident plane and raises the probability rate of generating high-energy γ-rays significantly by non-linear Compton scattering. As a result, 1.5×1013 γ photons with a mean energy of 9.2 MeV are obtained. These photons mainly point toward two directions and distribute in pulses with the duration of each pulse of ∼400 as. A brightness as high as 3.6×1025 photons/(mm2mrad2s0.1%BW) is obtained in one pulse. Compared to no positron observed for a plane target, a positron yield of 3.0×108 can be achieved in this new scheme.

Resolution of a discrepancy in the γ -ray emission probability from the β decay of Ceg137

We have deduced the emission probability of the 447-keV $\ensuremath{\gamma}$ ray from the $\ensuremath{\varepsilon}+{\ensuremath{\beta}}^{+}$ decay of $^{137}\mathrm{Ce}^{g}$ (9.0 h) relative to that of the 254-keV $\ensuremath{\gamma}$ ray from the $^{137}\mathrm{Ce}^{m}$ (34.4 h) decay in transient equilibrium. The time-dependent factor in transient equilibrium was applied following the Bateman equation for a radioactive decay chain. The isotope was produced via the $^{139}\mathrm{La}(p,3n)^{137}\mathrm{Ce}^{m,g}$ reaction by bombarding ${}^{\mathrm{nat}}\mathrm{La}$ with a proton beam from the 88-in. cyclotron at Lawrence Berkeley National Laboratory. $\ensuremath{\gamma}$-ray intensities were measured using an HPGe detector. The emission probability for the 447-keV $\ensuremath{\gamma}$ ray deduced in this work is 1.21(3) (that is 1.21 \ifmmode\pm\else\textpm\fi{} 0.03) per hundred parent decays, which differs significantly from an earlier published value of 2.24(10). We identify the source of this discrepancy to be an incorrect use of the time-dependent factor. Additionally, we have deduced the emission probability of the 504-keV $\ensuremath{\gamma}$ ray from the decay of $^{85}\mathrm{Y}^{g}$ (2.68 h) relative to that of the 232-keV $\ensuremath{\gamma}$ ray from the $^{85}\mathrm{Sr}^{m}$ (1.127 h) decay in transient equilibrium. The isotope was produced via the $^{86}\mathrm{Sr}(p,2n)^{85}\mathrm{Y}^{g}$ reaction by bombarding $^{86}\mathrm{SrCO}_{3}$ with a proton beam at the same facility. The study confirms the assumption of the time-dependent correction for recommending the emission probability of the 504-keV $\ensuremath{\gamma}$ ray in the literature. Our work highlights the importance of explicit description by authors of any time-dependent correction they have made when reporting $\ensuremath{\gamma}$-ray intensities for nuclides in transient equilibrium. The need and significance of accurate and precise decay data of $^{137}\mathrm{Ce}^{g}$ and $^{85}\mathrm{Y}^{g}$ in basic science and medicine is briefly outlined.

Targeted Brain Tumor Radiotherapy Using an Auger Emitter.

Glioblastoma multiforme is a highly aggressive form of brain cancer whose location, tendency to infiltrate healthy surrounding tissue, and heterogeneity significantly limit survival, with scant progress having been made in recent decades. 123I-MAPi (Iodine-123 Meitner-Auger PARP1 inhibitor) is a precise therapeutic tool composed of a PARP1 inhibitor radiolabeled with an Auger- and gamma-emitting iodine isotope. Here, the PARP inhibitor, which binds to the DNA repair enzyme PARP1, specifically targets cancer cells, sparing healthy tissue, and carries a radioactive payload within reach of the cancer cells' DNA. The high relative biological efficacy of Auger electrons within their short range of action is leveraged to inflict DNA damage and cell death with high precision. The gamma ray emission of 123I-MAPi allows for the imaging of tumor progression and therapy response, and for patient dosimetry calculation. Here we demonstrated the efficacy and specificity of this small-molecule radiotheranostic in a complex preclinical model. In vitro and in vivo studies demonstrate high tumor uptake and a prolonged survival in mice treated with 123I-MAPi when compared with vehicle controls. Different methods of drug delivery were investigated to develop this technology for clinical applications, including convection enhanced delivery and intrathecal injection. Taken together, these results represent the first full characterization of an Auger-emitting PARP inhibitor which demonstrate a survival benefit in mouse models of GBM and confirm the high potential of 123I-MAPi for clinical translation.

Primary standardization and determination of gamma ray emission intensities of Ho-166

The procedure followed by the Nuclear Metrology Laboratory (LMN) at the IPEN-CNEN/SP, in São Paulo, for the primary standardization of 166Ho is described. The activity of 166Ho was determined by the efficiency extrapolation technique applied to a 4πβ(PC)-γ coincidence system using a gas flow proportional counter in 4π geometry coupled to a 76 × 76 mm NaI(Tl) crystal. The results for the γ-rays intensities at 80.57 and 1379.45 keV were 0.0651(11) and 0.00904(11), respectively.