Bragg peak

{{Use American English|date = February 2019}} {{Short description|Path length of maximum energy loss of ionizing radiation}}{{For|use of this term to describe '''diffraction peaks''', also known as reflections|Bragg reflection}} [[Image:Bragg Curve for Alphas in Air-PT-en.svg|thumb|The Bragg curve of 5.49 MeV alphas in air has its peak to the right and is skewed to the left, unlike the x-ray beam below.]] The '''Bragg peak''' is a pronounced peak on the Bragg curve which plots the energy loss of ionizing radiation during its travel through matter. For protons, α-rays, and other ion rays, the peak occurs immediately before the particles come to rest. It is named after William Henry Bragg, who discovered it in 1903 using alpha particles from radium,<ref>{{cite book|last1=Charlie Ma|first1=C-M|last2=Lomax|first2=Tony|title=Proton and carbon ion therapy|date=2012|publisher=CRC Press|location=Boca Raton|isbn=9781439816073|page=4}}</ref><ref>{{cite journal | author = Bragg, W. H. | year = 1904 | title = LXXIII. On the absorption of α rays, and on the classification of the α rays from radium | journal = The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science | volume=8 | issue = 48 | pages=719–725 | doi=10.1080/14786440409463245| url = https://zenodo.org/record/2107492 }}</ref> and wrote the first empirical formula for ionization energy loss per distance with Richard Kleeman.<ref>{{cite journal | last1=Bragg | first1=William Henry | first2=Richard | last2=Kleeman | title=XXXIX. On the α particles of radium, and their loss of range in passing through various atoms and molecules | journal = The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science | url = https://scholar.archive.org/work/hg2frxomqfb6zin632345owdou/access/ia_file/crossref-pre-1909-scholarly-works/10.1080%252F14786440508562463.zip/10.1080%252F14786440509463378.pdf | doi = 10.1080/14786440509463378 | volume=10 | year = 1905 | issue=57 | pages=318–340}}</ref>

When a fast charged particle moves through matter, it ionizes atoms of the material and deposits a dose along its path. A peak occurs because the interaction cross section increases as the charged particle's energy decreases. Energy lost by charged particles is inversely proportional to the square of their velocity, which explains the peak occurring just before the particle comes to a complete stop.<ref>{{cite web|title=Bragg Curves and Peaks|url=https://www.bnl.gov/nsrl/userguide/bragg-curves-and-peaks.php|website=Brookhaven National Laboratory|accessdate=27 January 2016}}</ref> In the upper figure, it is the peak for alpha particles of 5.49 MeV moving through air. In the lower figure, it is the narrow peak of the "native" proton beam curve which is produced by a particle accelerator of 250 MeV. The figure also shows the absorption of a beam of energetic photons (X-rays) which is entirely different in nature; the curve is mainly exponential. [[Image:BraggPeak-en.svg|thumb|left|The dose produced by a native and by a modified proton beam in passing through tissue, compared to the absorption of a photon or x-ray beam|200px]] This characteristic of proton beams was first recommended for use in cancer therapy by Robert R. Wilson in his 1946 article, "Radiological Use of Fast Protons".<ref>{{Cite journal|last=Wilson|first=Robert R.|date=1946-11-01|title=Radiological Use of Fast Protons|url=https://pubs.rsna.org/doi/10.1148/47.5.487|journal=Radiology|volume=47|issue=5|pages=487–491|doi=10.1148/47.5.487|pmid=20274616|issn=0033-8419|url-access=subscription}}</ref> Wilson studied how the depth of proton beam penetration could be controlled by the energy of the protons. This phenomenon is exploited in particle therapy of cancer, specifically in proton therapy, to concentrate the effect of light ion beams on the tumor being treated while minimizing the effect on the surrounding healthy tissue.<ref name="Trikalinos (2009)">{{cite book|last=Trikalinos|first=TA|title=Particle Beam Radiation Therapies for Cancer [Internet]. Comparative Effectiveness Technical Briefs, No. 1|chapter=Results |year=2009|publisher=Agency for Healthcare Research and Quality (US)|location=Rockville (MD)|pages=ES1–ES5|url=https://www.ncbi.nlm.nih.gov/books/NBK44543/|display-authors=etal}}</ref>

The blue curve in the figure ("modified proton beam") shows how the originally monoenergetic proton beam with the sharp peak is widened by increasing the range of energies, so that a larger tumor volume can be treated. The plateau created by modifying the proton beam is referred to as the spread out Bragg Peak, or SOBP, which allows the treatment to conform to not only larger tumors, but to more specific 3D shapes.<ref>{{Cite journal|year=2011|url=https://iopscience.iop.org/article/10.1088/0031-9155/56/11/N01/meta|language=en|doi=10.1088/0031-9155/56/11/N01|pmid=21558588|last1=Jette|first1=D.|last2=Chen|first2=W.|title=Creating a spread-out Bragg peak in proton beams |journal=Physics in Medicine and Biology|volume=56|issue=11|pages=N131-8|bibcode=2011PMB....56N.131J |s2cid=37517481 |url-access=subscription}}</ref> This can be achieved by using variable thickness attenuators like spinning wedges.<ref>{{cite web|last1=Paganetti|first1=Harald|last2=Bortfeld|first2=Thomas|title=Proton Beam Radiotherapy - The State of the Art1|url=http://www.aapm.org/meetings/05AM/pdf/18-4016-65735-22.pdf|website=AAPM|accessdate=27 January 2016|pages=16}}</ref> Momentum cooling in cyclotron-based proton therapy facilities enables a sharper distal fall-off of the Bragg peak and the attainment of high dose rates.<ref>Maradia, V., Meer, D., Dölling, R. et al. Demonstration of momentum cooling to enhance the potential of cancer treatment with proton therapy. Nat. Phys. (2023). https://doi.org/10.1038/s41567-023-02115-2.</ref>

As shown in the plots above, there is a limited range for the particles in a material. The '''Bragg–Kleeman rule''' is a way to estimate a particle's range in a medium, serving as a tool in particle detection and dosimetry. The basic form of the rule is:

:<math>\frac{R_1}{R_2} = \frac{\rho_2 \sqrt{A_1}}{\rho_1 \sqrt{A_2}}</math>

where ''R''1 and ''R''2 are the ranges of two particles, ''ρ''1 and ''ρ''2 are the densities of the media they traverse, and ''A''1 and ''A''2 are the atomic weights of the particles.<ref>{{Cite journal |last=Ulmer |first=W. |date=2007-07-01 |title=Theoretical aspects of energy–range relations, stopping power and energy straggling of protons |url=https://www.sciencedirect.com/science/article/abs/pii/S0969806X07001314#:~:text=The%20Bragg%E2%80%93Kleeman%20rule%20R,approximation%20(CSDA)%20is%20assumed. |journal=Radiation Physics and Chemistry |volume=76 |issue=7 |pages=1089–1107 |doi=10.1016/j.radphyschem.2007.02.083 |bibcode=2007RaPC...76.1089U |issn=0969-806X|url-access=subscription }}</ref> {{clear}}

==References== <references />

== External links == * {{cite web |last1=Wagenaar |first1=Douglas |title=7.1.3 The Bragg Curve |url=http://www.med.harvard.edu/jpnm/physics/nmltd/radprin/sect7/7.1/7_1.3.html |website=Radiation Physics Principles |accessdate=27 January 2016 |date=1995 |url-status=dead |archiveurl=https://web.archive.org/web/20160301184458/http://www.med.harvard.edu/jpnm/physics/nmltd/radprin/sect7/7.1/7_1.3.html |archivedate=1 March 2016 }} * {{cite web|title=Bragg peak|url=http://www.oxfordreference.com/view/10.1093/oi/authority.20110803095523714|website=Oxford Reference|publisher=Oxford University Press|accessdate=27 January 2016}} *{{Cite journal|last=Hojo|first=Hidehiro|date=3 July 2017|title=Difference in the relative biological effectiveness and DNA damage repair processes in response to proton beam therapy according to the positions of the spread out Bragg peak|journal=Radiation Oncology|volume=12|issue=1|page=111|doi=10.1186/s13014-017-0849-1|pmid=28673358|pmc=5494883 |doi-access=free }} *{{Cite journal|last=Endo|first=Masahiro|date=20 October 2017|title=Robert R. Wilson (1914–2000): the first scientist to propose particle therapy—use of particle beam for cancer treatment|journal=Radiological Physics and Technology|volume=11|issue=1|pages=1–6|doi=10.1007/s12194-017-0428-z|pmid=29058267|s2cid=3526846|doi-access=free}} *{{Cite journal|last=Grun|first=Rebecca|date=10 January 2017|title=Systematics of relative biological effectiveness measurements for proton radiation along the spread out Bragg peak: experimental validation of the local effect model|url=https://iopscience.iop.org/article/10.1088/1361-6560/62/3/890/meta|access-date=19 March 2021|journal=Physics in Medicine and Biology|volume=62|issue=3|pages=890–908|doi=10.1088/1361-6560/62/3/890|pmid=28072575|bibcode=2017PMB....62..890G |s2cid=24855475 |url-access=subscription}}

{{DEFAULTSORT:Bragg Peak}} Category:Ionizing radiation Category:Experimental particle physics Category:Radiation therapy