positron

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Positron (e + )

classification
Elementary particle
fermion
lepton
properties
electric charge 1  e
(1.602 176 634 · 10 −19 C )
Dimensions 5.485 799 090 65 (16) · 10 −4   u
9.109 383 7015 (28) · 10 −31   kg
1  m e
Resting energy 0.510 998 950 00 (15)   MeV
Compton wavelength 2.426 310 238 67 (73) · 10 −12   m
magnetic moment −9.284 764 7043 (28) · 10 −24   J  /  D
g factor −2.002 319 304 362 56 (35)
gyromagnetic
ratio 		1.760 859 630 23 (53) 10 11  1 / ( s T )
Spin 1/2
average lifespan stable
Interactions weak
electromagnetic
gravitation

The positron ( Art word from posi tive charge and elec tron ), symbols , is an elementary from the group of leptons . It is the antiparticle of the electron with which it agrees in all properties except for the sign of the electric charge and the magnetic moment .

If a positron and an electron meet, a pair annihilation can occur. In an ideal vacuum in which there are no electrons, however, positrons are stable.

The positron was the first known antiparticle. Its existence was predicted by Paul AM Dirac in 1928 . Carl David Anderson discovered it experimentally in cosmic rays on August 2, 1932, and gave it his name. Since the quantum mechanical properties of all electrons, apart from charge and helicity , are the same, the term pair positron - negatron was proposed for the two variants of the electron. However, the term Negatron has not caught on and is only used occasionally in the literature today.

Emergence

Positrons are created

In a normal environment, positrons "disappear" within a very short time through mutual annihilation with electrons, usually with the emission of two gamma quanta . Annihilation can be preceded by the formation of a positronium atom. Only in a very good vacuum can positrons be stored using magnetic fields.

Applications

Applications of positrons outside of basic physical research are based on the special, easily identifiable radiation of pair annihilation. Positron emission tomography (PET) in particular is an important imaging method in modern medical technology . In this case, the patient is administered a positron-emitting radiopharmaceutical , namely a substance that occurs in the human metabolism (e.g. glucose ). A β + -radioactive atom is coupled to the molecule of this substance either in addition to or instead of a non-radioactive atom. Glucose is metabolized to a greater extent by tissues with high energy requirements such as tumors or the brain, so it is more concentrated there than in other regions. The gamma quanta that arise in pairs during the positron-electron annihilation are detected with detectors outside the body. Since the quanta of a pair always fly away in opposite directions, an accumulation of the radiating glucose molecules can be easily localized and their concentration can be visualized.

In nuclear medicine , it should be noted that the radioactive nuclide is long enough on the one hand to be incorporated into a biomolecule and brought to the patient from the manufacturing laboratory (usually a cyclotron system), but on the other hand short-lived enough to enable imaging during the measurement , but then no longer unnecessarily exposing the patient to radiation. The main tracer used in PET is FDG-18 , in which a 19 F atom is replaced by a radioactive 18 F atom (half-life 109.77 min).

literature

  • Lisa Randall: Hidden Universes: A Journey into Extra-Dimensional Space . 4th edition. Fischer, Frankfurt 2006, ISBN 3-10-062805-5 .

Web links

Wiktionary: Positron  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ CODATA Recommended Values. National Institute of Standards and Technology, accessed July 31, 2019 . Elementary charge in C (exact).
  2. ↑ CODATA Recommended Values. National Institute of Standards and Technology, accessed July 31, 2019 . Electron mass in u . The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
  3. ↑ CODATA Recommended Values. National Institute of Standards and Technology, accessed July 31, 2019 . Electron mass in kg . The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
  4. ↑ CODATA Recommended Values. National Institute of Standards and Technology, accessed July 31, 2019 . Electron mass in MeV / c 2 . The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
  5. ↑ CODATA Recommended Values. National Institute of Standards and Technology, accessed July 31, 2019 . Compton wavelength. The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
  6. ↑ CODATA Recommended Values. National Institute of Standards and Technology, accessed July 31, 2019 . Magnetic moment. The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
  7. ↑ CODATA Recommended Values. National Institute of Standards and Technology, accessed July 31, 2019 . g factor. The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
  8. ↑ CODATA Recommended Values. National Institute of Standards and Technology, accessed July 31, 2019 . Gyromagnetic ratio. The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
  9. ^ PAM Dirac: The Quantum Theory of the Electron . In: Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character . A, no. 778 , 1928, pp. 610-624 , doi : 10.1098 / rspa.1928.0023 ( online ).
  10. ↑ CD Anderson: The Positive Electron . In: Physical Review . tape 43 , no. 6 , 1933, pp. 491–494 , doi : 10.1103 / PhysRev.43.491 ( online ).