I simulated a calibration laboratory using MCNP code. The source in the irradiator is Co-60, emitting two gamma rays (1.17 and 1.33 MeV) through decay. The output provided me with the values of dose equivalent ambient at the calibration points. Now, I need to determine the dose equivalent ambient rate with the corrected activity of my source. I followed a similar methodology used for Cs-137, which I successfully validated with experimental data from that laboratory. However, for Co-60, it is not yielding the expected results. I have not yet identified the issue with my analysis.
To obtain H*(10), the chosen tally was F5, and the conversion factors from ICRP 74 were applied using DE/DF on the data card. The input yields H(10) per NPS.
Attached are photos of my methodology to aid in understanding my question.
The methodology of the work, whose photo is attached, was also tested; however, it did not yield results that could be validated by experimental data.
(This work is referenced with the attached photo of its cover)
Hi Sara, before going through the attachments I have to ask: what difference you see between measurement and calculation? And then, I would ask you to simply show your tally result, the Co-60 activity and a simple line showing how you did the tally normalization (going from the F5+DCF units to rad/hr or mSv/hr). The handwritten notes are a bit hard to follow. In fact, I am not sure what NPS is; is it the total histories in MCNP? If so, those matter only on the statistics. Sorry, I cannot help you right away.
Dear Dimitrios Kontogeorgakos,
I possess empirical data obtained from laboratory measurements, pertinent to the simulation I am currently conducting. The laboratory facility incorporates an irradiation apparatus housing two distinct radiation sources, specifically Cobalt-60 and Cesium-137. Within the simulation, I put the tally for flux measurement at a specific point on the detector, denoted as F5. Further, I have availed myself of the guidance offered by Table A.21 of the ICRP Publication 74 "Conversion Coefficients for use in Radiological Protection against External Radiation" to facilitate the conversion of the aforementioned flux measurements into equivalent ambient dose values, expressed as H*(10). Consequently, the computed outcome yields a definitive value of H*(10) in pSv per emitted photon originating from the respective source (NPS).
Herein lies the pivotal juncture that delineates my quandary: The Cesium-137 source emits one gamma ray per radioactive decay, while the Cobalt-60 source emits two distinct gammas. The procedural methodology adopted for Cesium-137 entails an approach whereby activity is defined as the quotient of disintegrations per unit of time. The count of disintegrations, in this context, corresponds to the number of photons emitted from the source (referred to as NPS). Given my objective of ascertaining an ambient dose equivalent rate, it is imperative to normalize this value concerning time, because I want a dose rate. To this end, time is characterized by the ratio of disintegrations to activity, the latter being the product of NPS divided by the activity. Consequently, the resultant output is represented as (H*(10) pSv / NPS) * NPS * 1 / t. This expression can be rephrased as H*(10) / NPS * NPS * A / NPS, which can be further simplified to H*(10) / NPS * A, expressed in mSv per hour.
However, a discrepancy emerges when applying the same methodology to Cobalt-60. I initially attempted to treat Cobalt-60 as having a single average emission of 1.25 MeV, yet the results mirrored those obtained with the inclusion of both distinct emissions of Co-60 (1.17 and 1.33 MeV). Regrettably, the methodology did not yield applicable results for Cobalt-60. I surmise that a more nuanced approach is requisite in addressing the issue of dual emissions; however, I have yet to arrive at a suitable solution.
Kindly find attached the requested data.
Once again, I am sincerely grateful for your invaluable guidance and insight.
Best Regards,
Hi Sara, although I have not completely understood what the attached picture shows here are my thoughts. The "output (pSv/NPS) are the tally results with the DF/DE multiplier (to get the ambient dose rate from ICRP-74). The H*(10) results are the tally results after normalization. If that's true, then to get 1.5e+9 pSv/h from 3.41e-5 pSv/NPS, you have multiplied by 4.4e+13, which is the disintegrations per hour and not the photons per hour (unless, I am not reading the data on the png file correctly). Basically, 1.5e+9 pSv/h needs to be multiplied by 2 (2 photons emitted per Co-60 disintegration). Is the "Taxa de Equivalente de Dose Ambiente)" the measured values? If so, then by multiplying those by 2 will get you closer to them. What was the MCNP to measured ratio for Cs-137?