Q.15
ICAR AIEEA PG Physical Science 14 July 2023 Shift-1 Paper (Q.39)
MCQ (Single Correct)
English हिन्दी
The process by which neutrons lose their kinetic energy through elastic collisions in the soil is known as:
वह प्रक्रिया जिसके द्वारा मृदा में प्रत्यास्थ संघट्टों (इलास्टिक कोलिजन) के माध्यम से न्यूट्रॉन अपनी गतिज ऊर्जा खो देते हैं, कहलाती है:
EXPLANATION Soil Water Retention, Energy Concepts, Soil Water Potential, Retention Curves (pF) and Hysteresis

Physics of the Neutron Moisture Meter:

  • radioactive source (e.g., Americium-241/Beryllium, $^{241}\text{Am-Be}$) emits high-energy fast neutrons ($4-5\text{ MeV}$) into the soil.
  • When fast neutrons collide elastically with nuclei of similar mass—specifically hydrogen atoms ($^1H$) primarily present in soil water molecules ($H_2O$)—they transfer substantial momentum and bounce off with dramatically reduced energy.
  • This process of slowing down fast neutrons to low-energy thermal / slow neutrons ($\sim 0.025\text{ eV}$) in equilibrium with the thermal motion of the medium is technically termed 'Thermalization' (or moderation).
  • detector ($^3He$ or $BF_3$ tube) counts the flux of thermalized neutrons, which is directly proportional to the volumetric water content.
Therefore, the process is known as Thermalization.

न्यूट्रॉन नमी मापी (Neutron Probe) का सिद्धांत:

  • स्रोत ($^{241}\text{Am-Be}$) से तीव्र गति वाले उच्च ऊर्जा न्यूट्रॉन मृदा में छोड़े जाते हैं।
  • जब ये तीव्र न्यूट्रॉन मृदा जल ($H_2O$) में उपस्थित हाइड्रोजन परमाणुओं ($^1H$) के समान द्रव्यमान वाले नाभिकों से टकराते हैं, तो अपनी गतिज ऊर्जा खो देते हैं।
  • तीव्र न्यूट्रॉनों का धीमे तापीय न्यूट्रॉनों (Thermal neutrons) में बदलने की इस प्रक्रिया को 'थर्मलाइजेशन' (Thermalization / तापीयकरण) कहा जाता है।
अतः सही उत्तर थर्मलाइजेशन है।
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