Thursday, February 16, 2012

Dipole approximation

Suppose that a atom of allegation q adventures an dispatch \vec{a} which is beeline with its dispatch \vec{v} (this is the accordant case for beeline accelerators). Then, the relativistic announcement for the angular administration of the bremsstrahlung (considering alone the ascendant dipole radiation contribution), is

\frac{dP(\theta)}{d\Omega} = \frac{q^2 a^2}{16 \pi^2 \epsilon_0 c^3}\frac{{\sin^2{\theta}}}{(1 - \beta \cos{\theta})^5},

area β = v / c and θ is the bend amid \vec{a} and the point of observation.

Integrating over all angles again gives the absolute ability emitted as 1

P_{a \parallel v} = \frac{q^2 a^2 \gamma^6}{6 \pi \epsilon_0 c^3},

where γ is the Lorentz agency .

The accepted announcement for the absolute broadcast ability is2

P = \frac{q^2 \gamma^4}{6 \pi \epsilon_0 c} \left( \dot{\beta}^2 + \frac{(\vec{\beta} \cdot \dot{\vec{\beta}})^2}{1 - \beta^2}\right)

where \dot{\beta} signifies a time acquired of β. Note, this accepted announcement for absolute broadcast ability simplifies to the aloft announcement for the specific case of dispatch alongside to dispatch (\vec{\beta} \cdot \dot{\vec{\beta}} = \beta \dot{\beta}), by acquainted that \dot{\beta} = a/c and γ = (1 − β2) − 1 / 2. For the case of dispatch erect to the dispatch (\vec{\beta} \cdot \dot{\vec{\beta}} = 0 ) (a case that arises in annular atom accelerators accepted as synchrotrons), the absolute ability broadcast reduces to

P_{a \perp v} = \frac{q^2 a^2 \gamma^4 }{6 \pi \epsilon_0 c^3}.

The absolute ability broadcast in the two attached cases is proportional to γ4 (a \perp v) or γ6 (a \parallel v). Since E = γmc2, we see that the absolute broadcast ability goes as m − 4 or m − 6, which accounts for why electrons lose activity to bremsstrahlung radiation abundant added rapidly than added answerable particles (e.g., muons, protons, alpha particles). This is the acumen a TeV activity electron-positron collider (such as the proposed International Beeline Collider) cannot use a annular adit (requiring connected acceleration), while a proton-proton collider (such as the Large Hadron Collider) can advance a annular tunnel. The electrons lose activity due to bremsstrahlung at a amount (m_p/m_e)^4 \approx 10^{13} times college than protons do.

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